Add "bundle check || sudo bundle install" into prophet.rb

These commands ensure that pull requests which add new gems to the
dependencies won't fail when Prophet executes the tests.

I used "sudo" in case of "bundle install" to avoid possible password
prompts. It's not very clean, but we use it already in scripts on
Jenkins without any negative consequences.
Merge pull request #112 from SUSE/bundle-in-prophet-rb

Add "bundle check || sudo bundle install" into prophet.rb