(PUP-2842) Limit platforms on which we install ruby-prof

The ruby-prof gem is not applicable on all platforms.  This
commit simply adds a :platform restriction to the Gemfile.
Merge pull request #2808 from cprice404/bug/master/PUP-2842-limit-platforms-for-ruby-prof

(PUP-2842) Limit platforms on which we install ruby-prof