Allow app to specify BUNDLE_GEMFILE environment variable about ruby-buildpack

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. You can view the current status of your issue at: https://www.pivotaltracker.com/story/show/73334472.

[squeedee]

We have resolved the issue in this commit: f46da81

If you want to try it, you can reference the URL:

$ cf push appname -b https://github.com/cloudfoundry/buildpack-ruby.git

Or if you like, you can create an admin buildpack:

git clone https://github.com/cloudfoundry/buildpack-ruby.git
cd buildpack-ruby
./bin/package offline
 cf create-buildpack admin_ruby_buildpack ruby_buildpack.zip 1