Specify sass-rails version in Gemfile

Fixes #325

Bundler could not find compatible versions for gem "sprockets":
  In Gemfile:
    sass-rails (>= 0) ruby depends on
      sprockets (~> 2.0.0) ruby

    rails (~> 4.1.0) ruby depends on
      sprockets-rails (~> 2.0) ruby depends on
        sprockets (2.12.1)

Bundler could not find compatible versions for gem "sprockets-rails":
  In Gemfile:
    sass-rails (>= 0) ruby depends on
      sprockets-rails (~> 2.0.0) ruby

    rails (~> 4.1.0) ruby depends on
      sprockets-rails (2.1.3)

Bundler could not find compatible versions for gem "railties":
  In Gemfile:
    sass-rails (>= 0) ruby depends on
      railties (~> 3.1.0) ruby

    rails (~> 4.1.0) ruby depends on
      railties (4.1.0)

templates/Gemfile.erb

@@ -15,7 +15,7 @@ gem 'pg'
 gem 'rack-timeout'
 gem 'rails', '~> 4.1.0'
 gem 'recipient_interceptor'
-gem 'sass-rails'
+gem 'sass-rails', '~> 4.0.3'
 gem 'simple_form'
 gem 'title'
 gem 'uglifier'