Fix Bundler::RubyVersionMismatch error when invoking heroku command from within Bundler.

As described here: https://github.com/bundler/bundler/issues/2355

In our case, to reproduce the issue I locked the Gemfile of a new Rails 4.0.2 app to Ruby 2.0.0, deployed to Heroku, and then ran the generator with `rails generate honeybadger --heroku`.