add gem name for use with bundler

[calebhearth]

I feel like at this point most people who will be using something like Formulaic, which isn't super entry-level since you need knowledge of rspec, rails, and to have felt the pain of using Capybara, know how to install gems. Did this cause a problem for you?

[acrogenesis]

I had to look into formulaic.gemspec to make sure the gem was not named different. e.g. formulaic-thoughtbot

[calebhearth]

Fixed in be71be7