Fix: ruby app fails to push
Fix: ruby app fails to push

Update gemfile to use ruby version 2.1.7.

As of v1.6.4 of the [buildpack](https://github.com/cloudfoundry/ruby-buildpack/releases) (Aug 2015), ruby 2.1.5 support has been removed
krujos/cf-hello-world-sample-apps,krujos/cf-hello-world-sample-apps,krujos/cf-hello-world-sample-apps,krujos/cf-hello-world-sample-apps,krujos/cf-hello-world-sample-apps