Prepare for installation via Bundler

This commit does the following things:

- Adds a Rake task "vendor" which moves the generated Gem into
  vendor/bundle so it can be used as a Bundler source
- Adds a .ruby-version file (currently 2.1.1)
- Updates Gemfile.lock
- Adds a new Gemfile.run that can be used later by deployment tools
  (BUNDLE_GEMFILE=Gemfile.run bundle install --deployment)