Calling "brew" in Ruby system call and bundler error - ruby

Question

I'm trying to write a rubygem that does some stuff with homebrew. I'm calling on brew to do some stuff using system(). When I include certain gems in using bundler, I get some errors doing things inside system().
As an example, a basic Gemfile with rake:
~/tmp/foo cat Gemfile
source 'https://rubygems.org'
gem 'rake'
Inside irb after running a bundle install:
irb(main):001:0> system("brew", "info", "ack")
Could not find rake-12.0.0 in any of the sources
Run `bundle install` to install missing gems.
=> false
I can do a bundle install inside the call and receive the same results:
rb(main):007:0> system("bundle", "install")
Using rake 12.0.0
Using bundler 1.14.6
Bundle complete! 1 Gemfile dependency, 2 gems now installed.
Bundled gems are installed into /Users/laura/.bundle.
=> true
irb(main):008:0> system("brew", "version")
Could not find rake-12.0.0 in any of the sources
Run `bundle install` to install missing gems.
=> false
Without the rake gem included I do not receive any errors:
~/tmp/foo cat Gemfile
source 'https://rubygems.org'
gem 'git'
~/tmp/foo bundle install
Fetching gem metadata from https://rubygems.org/.
Fetching version metadata from https://rubygems.org/
Resolving dependencies...
Using git 1.3.0
Using bundler 1.14.6
Bundle complete! 1 Gemfile dependency, 2 gems now installed.
Bundled gems are installed into /Users/surminus/.bundle.
~/tmp/foo
~/tmp/foo irb
irb(main):002:0> system("brew", "info", "ack")
ack: stable 2.16, HEAD
Search tool like grep, but optimized for programmers
https://beyondgrep.com/
/usr/local/Cellar/ack/2.14 (4 files, 184.7KB)
Built from source on 2016-12-16 at 16:24:07
/usr/local/Cellar/ack/2.16 (4 files, 190.7KB) *
Built from source on 2017-03-17 at 20:25:45
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/ack.rb
=> true
I know it's probably related to some conflict between homebrew, rake and bundler, but I can't for the life of me figure out what's going on. I only need to rake tasks for some development specific tasks so I can remove it, but I'm interested to find out why this occurs, or if indeed calling brew this way is $bad.
EDIT: I was asked for output of gem list --local in the bundle directory. Seems to be system gems. With bundle exec:
~/tmp/foo bundle exec gem list --local
*** LOCAL GEMS ***
bundler (1.14.6)
rake (12.0.0)
Without bundle exec:
~/tmp/foo gem list --local
*** LOCAL GEMS ***
activemodel (5.0.1)
activesupport (5.0.1)
addressable (2.5.0)
bigdecimal (1.2.8)
bundler (1.14.6, 1.14.5, 1.13.6)
CFPropertyList (2.2.8)
coderay (1.1.1)
commander (4.4.2)
concurrent-ruby (1.0.3)
did_you_mean (1.0.0)
diff-lcs (1.3, 1.2.5)
domain_name (0.5.20161129)
facter (2.4.6 ruby universal-darwin)
fileutils (0.7)
git (1.3.0)
hiera (1.3.4)
highline (1.7.8)
http-cookie (1.0.3)
i18n (0.7.0)
io-console (0.4.5)
json (2.0.2, 1.8.3)
json_pure (2.0.1, 1.8.3)
metaclass (0.0.4)
method_source (0.8.2)
mime-types (3.1)
mime-types-data (3.2016.0521)
minitest (5.10.1, 5.8.3)
mocha (1.2.1, 1.1.0)
myosx (0.2.2, 0.2.0, 0.1.0)
net-telnet (0.1.1)
netrc (0.11.0)
oauth (0.5.1)
power_assert (0.2.6)
pry (0.10.4)
psych (2.0.17)
public_suffix (2.0.4)
puppet (3.8.7)
puppet-lint (1.1.0)
puppet-syntax (2.3.0, 2.1.0)
puppetlabs_spec_helper (1.1.1)
rake (12.0.0, 11.1.2, 10.5.0, 10.4.2)
rdoc (4.2.1)
rest-client (2.0.0)
rmagick (2.16.0)
rspec (3.5.0, 3.4.0)
rspec-core (3.5.4, 3.4.4)
rspec-expectations (3.5.0, 3.4.0)
rspec-mocks (3.5.0, 3.4.1)
rspec-puppet (2.4.0)
rspec-support (3.5.0, 3.4.1)
ruby-trello (1.6.0)
slop (3.6.0)
test-unit (3.1.5)
thread_safe (0.3.5)
tzinfo (1.2.2)
unf (0.1.4)
unf_ext (0.0.7.2)