setup bundler from TM_PROJECT_DIRECTORY so it finds a gemfile when BUNDLE_GEMFILE isn't set

Because Textmate commands are run from dynamically generated files in /tmp Bundler.setup does not find the proper Gemfile because its searching in /tmp instead of the project folder. This change just runs Bundler.setup from the current project directory, so if BUNDLE_GEMFILE isn't set it falls back to the correct default.

There is a stack trace that set me down the path to this commit, but instead of pasting it here I think it suffices to say that bundler threw Bundler::GemfileNotFound and it traced back to this call to Bundler.setup.