Rationalize Gemfile

All tests pass, except the missing home#index (which was failing already). `rails s` runs.

Per your request, this despecifies (unspecifies) the gem version numbers.

Moves Thin to a production group, which moving (I later found out) doesn't change the behavior.

Includes 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 (included) lock the first-digit, major release numbers. For instance, `gem 'mongoid', '~> 3.0'`, protects us from (feature-breaking) mongoid 4.x.

Increases legibility, over the Rails Composer-derived Gemfile, by alphabetizing.

Of course, under the GitHub Files Changed tab, click Gemfile.lock to see the actual gem updates.