Update current gem defaults

Provide fewer options and more opinions:

* Add assets group to Gemfile.
* Add jQuery to Gemfile.
* Add Postgres to Gemfile.
* Add Rails to Gemfile.
* Add Strong Parameters to Gemfile.
* Alphabetize existing gems.
* Make Capybara Webkit a default.
* Remove Clearance.
* Remove Flutie.
* Remove Paperclip.
* Replace Formtastic with Simple Form.
* Specify minimum version of Capybara Webkit.
* Specify minimum version of Rails.