The setup script relies on bundled gems 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.

bin/setup

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