Merge branch 'setup-needs-bundler' into 'master'

bin/setup needs bundler too

By adding `require 'bundler/setup'` we remove the need for `bundle exec`.
This is already done in the other scripts in bin/; this change adds the
missing require to bin/setup.

See merge request !13

bin/setup

 #!/usr/bin/env ruby
+require 'bundler/setup'
 require_relative '../lib/setup'
 GitlabCi::Setup.new