Error with Gitlab: Sidekiq; gemfile syntax error-Collection of common programming errors

I’ve been breaking my head over this error for hours now, and still haven’t found a fix. When I run sidekiq using sudo -u root -H RAILS_ENV=production script/background_jobs start I’m getting this error in the sidekiq.log:

Gemfile syntax error:
/home/website/git/gitlab/Gemfile:20: syntax error, unexpected ':', expecting $end
gem "mysql2", group: :mysql

I’ve installed Ruby 2.1.0 and am running CentOS. I’ve seen other posts with this error, and their problem was that they were running Ruby 1.8, however, I’m running Ruby 2.1.0 and also getting this error.

Any help is greatly appreciated.

EDIT: My gemfile http://pastebin.com/T5z4GZ3a

bundle returns

Your bundle is complete!
Gems in the groups development, test, postgres, puma and aws were not installed.
It was installed into ./vendor/bundle

EDIT2: My background_jobs script http://pastebin.com/kdicTFqk

bundle show bundler returns

2.1.0
/usr/local/rvm/gems/ruby-2.1.0@global/gems/bundler-1.5.2

EDIT3: I think I have found the problem, not sure how to solve it though. I added p RUBY_VERSION as first line of my Gemfile, then I changed every line written like this: group: :mysql to this :group => :mysql (because that is working on older versions).

However, when I run Sidekiq now, this shows up in the sidekiq.log:

"1.8.7"
bundler: command not found: sidekiq
Install missing gem executables with `bundle install`

I am really, really confused now. Here it clearly says my Ruby version is 1.8.7, but that’s not right. ruby -v or rvm list rubies only show that Ruby 2.1.0 is installed, and set as default.