Add support for bundled rake

[mlarraz]

Projects using Gemfiles are always tied to a specific version of rake.
By default, calling rake will use whatever the highest version of rake installed is.
If this version conflicts with the one in the Gemfile.lock, the rake task will abort with

rake aborted!
Gem::LoadError: You have already activated rake 10.4.1, but your Gemfile requires rake 10.3.2. Prepending `bundle exec` to your command may solve this.

If a project has a Gemfile, we can assume they will want to run bundle exec rake, not rake.