Fix GH #4749. Remove branch options, because edge is not 3-2-stable.

[kennyj]

In 3-2-stable, when we run rails new app --edge, we see the following lines in Gemfile.

  gem 'sass-rails',   :git => 'git://github.com/rails/sass-rails.git', :branch => '3-2-stable'
  gem 'coffee-rails', :git => 'git://github.com/rails/coffee-rails.git', :branch => '3-2-stable'

I think that this is a problem.

Bundler could not find compatible versions for gem "railties":
  In Gemfile:
    sass-rails (>= 0) ruby depends on
      railties (~> 3.2.0) ruby

    rails (>= 0) ruby depends on
      railties (4.0.0.beta)

Please see #4749
Please see also kennyj@8bf3c6d (before rebasing)