Downgrading Ruby Version for my Rails App - ruby

Question

I'm trying to deploy my app 'project'. It's written in Ruby using Rails and it says I need to downgrade Ruby from 2.5.1 to 2.4.0. I've tried a mix of tinkering with my Gemfile and some RVM commands in my Terminal but nothing seems to work. SOS pls send help!
I changed the ruby v in the Gemfile to "2.4.0" and when I enter the directory it notifies me that:
Using: /Users/me/.rvm/gems/ruby-2.4.0#rails3
Then I tried to run "rails s" and receive:
Ignoring nokogiri-1.8.3 because its extensions are not built. Try: gem pristine nokogiri --version 1.8.3
/Users/me/.rvm/gems/ruby-2.4.0#rails3/gems/railties-5.2.0/lib/rails/app_loader.rb:53: warning: Insecure world writable dir /usr/local/bin in PATH, mode 040777
Ignoring bcrypt-3.1.12 because its extensions are not built. Try: gem pristine bcrypt --version 3.1.12
Ignoring bindex-0.5.0 because its extensions are not built. Try: gem pristine bindex --version 0.5.0
Ignoring bootsnap-1.3.0 because its extensions are not built. Try: gem pristine bootsnap --version 1.3.0
Ignoring byebug-10.0.2 because its extensions are not built. Try: gem pristine byebug --version 10.0.2
Ignoring ffi-1.9.25 because its extensions are not built. Try: gem pristine ffi --version 1.9.25
Ignoring msgpack-1.2.4 because its extensions are not built. Try: gem pristine msgpack --version 1.2.4
Ignoring nio4r-2.3.1 because its extensions are not built. Try: gem pristine nio4r --version 2.3.1
Ignoring nokogiri-1.8.3 because its extensions are not built. Try: gem pristine nokogiri --version 1.8.3
Ignoring puma-3.11.4 because its extensions are not built. Try: gem pristine puma --version 3.11.4
Ignoring sqlite3-1.3.13 because its extensions are not built. Try: gem pristine sqlite3 --version 1.3.13
Ignoring websocket-driver-0.7.0 because its extensions are not built. Try: gem pristine websocket-driver --version 0.7.0
/Users/brianna/.rvm/rubies/ruby-2.4.0/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- bundler (LoadError)
from /Users/me/.rvm/rubies/ruby-2.4.0/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /Users/me/proj/bin/spring:8:in `<top (required)>'
from bin/rails:3:in `load'
from bin/rails:3:in `<main>'

Answer 1

As per the description and logs shared it seems like you have changed the ruby version but haven't build the gemset over the modified version.
Below mentioned log trail implies that it could not find the bundler for the specific ruby version(2.4.0)
/Users/brianna/.rvm/rubies/ruby-2.4.0/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- bundler (LoadError)
Try following steps:
1) Install bundler corresponding to ruby 2.4.0 using
gem install bundler
2) Now, either delete the Gemfile.lock and run the following command:
bundle install
Or try running
bundle update
3) Note that if there are any gems specified with a specific version that are compatible with ruby >=2.5, then you may have to either remove the version number of specify the version which is compatible with ruby 2.4.0
Hope it helps!!

Answer 2

You are trying to instal Nokogiri gem which required that you first install some dependent libraries and build on your local machine some time. In order to do so you may follow following guide to install nokogiri.
Nokogiri Installation
Sometime it become typical to install nokogiri on some Enviornment