Rationalize Gemfile

All tests pass, except the missing home#index (which already was failing).

Per your request, despecifies the gem version numbers.

Increases legibility, from the Rails Composer-derived Gemfile, by alphabetizing.

Uses upper-bound, pessimistic version constraints, per Yehuda Katz' [article](http://yehudakatz.com/2010/08/21/using-considered-harmful-or-whats-wrong-with/) on how to protect ourselves (with them).

The pessimistic version constraints here lock in first-digit, major release numbers, as in, `gem 'mongoid', '~> 3.0'`, which protects us from (feature-breaking) mongoid 4.x.

Moves Thin to a production group.

Includes `bundle update`.