Update Rails to 4.2.0

0.0.3 of rails_server_static_assets includes a necessary change
for Suspenders to work with Rails 4.2.0:

https://github.com/heroku/rails_serve_static_assets/pull/7
Configure simple_form
Do not require bundler-audit in Gemfile

Move `bundler-audit` to the `development` gem group only.