`require': cannot load such file -- HTTParty (LoadError) - ruby

Question

First off I'm a Newbie in regards to Ruby, and I'm following the simple tutorial on https://www.distilled.net/resources/web-scraping-with-ruby-and-nokogiri-for-beginners/
I have a test.rb file that contains the following:
require 'rubygems'
require 'Nokogiri'
require 'HTTParty'
require 'Pry'
require 'csv'
# this is how we request the page we're going to scrape
page = HTTParty.get('https://newyork.craigslist.org/search/pet?s=0')
Pry.start(binding)
When I try to run the ruby test script, I get the following error:
pjw#ubuntu:~/ruby/test$ ruby test.rb
/home/pjw/.rvm/rubies/ruby-2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- HTTParty (LoadError)
from /home/pjw/.rvm/rubies/ruby-2.3.0/lib/ruby/site_ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from test.rb:1:in `<main>'
I've verified HTTParty works properly by running:
httparty "https://newyork.craigslist.org/search/pet?s=0"
The problem seems to be the require statement. If I mix the order or the required gems, the first one gives the same error as above. I've searched online and can't seem to find what I'm missing...
in `require': cannot load such file -- httparty (LoadError) - (No Answer)
kernel_require.rb:55:in `require': cannot load such file error - (bundle exec ruby test.rb "Could not locate Gemfile or .bundle/ directory")
Unable to run .rb files on mac - getting /.rbenv/versions/2.2.3/lib/ruby/site_ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require' - (GEMS already Installed)
pjw#ubuntu:~/ruby/test$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.1 LTS
Release: 16.04
Codename: xenial
pjw#ubuntu:~/ruby/test$ ruby -v
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-linux]
pjw#ubuntu:~/ruby/test$ gem list
*** LOCAL GEMS ***
actioncable (5.0.0.1)
actionmailer (5.0.0.1)
actionpack (5.0.0.1)
actionview (5.0.0.1)
activejob (5.0.0.1)
activemodel (5.0.0.1)
activerecord (5.0.0.1)
activesupport (5.0.0.1)
addressable (2.5.0)
arel (7.1.4)
autoprefixer-rails (6.5.3, 6.5.2, 6.5.1)
bcrypt (3.1.11)
bigdecimal (default: 1.2.8)
bootstrap (4.0.0.alpha5)
bootstrap-datepicker-rails (1.6.4.1)
bootstrap-sass (3.3.7)
builder (3.2.2)
bundler (1.13.6)
bundler-unload (1.0.2)
byebug (9.0.6)
cancancan (1.15.0)
coderay (1.1.1)
coffee-rails (4.2.1)
coffee-script (2.4.1)
coffee-script-source (1.10.0)
commonjs (0.2.7)
concurrent-ruby (1.0.2)
connection_pool (2.2.1, 2.2.0)
dashing-rails (2.5.0)
debug_inspector (0.0.2)
devise (4.2.0)
did_you_mean (1.0.2, 1.0.0)
erubis (2.7.0)
execjs (2.7.0)
executable-hooks (1.3.2)
ffi (1.9.14)
figaro (1.1.1)
gem-wrappers (1.2.7)
geocoder (1.4.0)
globalid (0.3.7)
gravatarify (3.1.1)
httparty (0.14.0)
i18n (0.7.0)
io-console (0.4.6, default: 0.4.5)
jbuilder (2.6.0)
jquery-rails (4.2.1)
json (2.0.2, default: 1.8.3)
less (2.6.0)
less-rails (2.8.0)
listen (3.1.5, 3.0.8)
loofah (2.0.3)
mail (2.6.4)
method_source (0.8.2)
mime-types (3.1)
mime-types-data (3.2016.0521)
mini_portile2 (2.1.0)
minitest (5.9.1, 5.8.3)
multi_json (1.12.1)
multi_xml (0.5.5)
net-telnet (0.1.1)
nio4r (1.2.1)
nokogiri (1.6.8.1)
orm_adapter (0.5.0)
power_assert (0.3.1, 0.2.6)
pry (0.10.4)
psych (2.1.1, default: 2.0.17)
public_activity (1.5.0)
public_suffix (2.0.4)
puma (3.6.0)
rack (2.0.1)
rack-test (0.6.3)
rails (5.0.0.1)
rails-dom-testing (2.0.1)
rails-html-sanitizer (1.0.3)
rails_serve_static_assets (0.0.5)
rails_stdout_logging (0.0.5)
railties (5.0.0.1)
rake (11.3.0, 10.4.2)
rb-fsevent (0.9.8)
rb-inotify (0.9.7)
rdoc (5.0.0, default: 4.2.1)
redis (3.3.1)
responders (2.3.0)
ruby_dep (1.5.0)
rubygems-bundler (1.4.4)
rubygems-update (2.6.8)
rufus-scheduler (3.2.2)
rvm (1.11.3.9)
sass (3.4.22)
sass-rails (5.0.6)
simple_calendar (2.2.0)
simple_form (3.3.1)
slop (4.4.1, 3.6.0)
spring (2.0.0)
spring-watcher-listen (2.0.1)
sprockets (3.7.0)
sprockets-rails (3.2.0)
sqlite3 (1.3.12)
table_print (1.5.6)
test-unit (3.2.2, 3.1.5)
thor (0.19.1)
thread_safe (0.3.5)
tilt (2.0.5)
turbolinks (5.0.1)
turbolinks-source (5.0.0)
twitter-bootstrap-rails (3.2.2)
tzinfo (1.2.2)
uglifier (3.0.3, 3.0.2)
warden (1.2.6)
web-console (3.4.0, 3.3.1)
websocket-driver (0.6.4)
websocket-extensions (0.1.2)
wunderground (1.2.0)

Answer 1

Try simply this. It's all downcase
require 'httparty'

Answer 2

ubuntu#ip-172-31-19-199:~/ruby$ sudo apt install ruby-httparty
Install ruby-httparty and you won't get the error.