Lower the coffee-script-source dependency version

There is an issue with the latest version of `coffee-script-source`
outlined in the following issue:

    jashkenas/coffeescript#3829

When the `react-rails` gemspec specifies a `~>1.9` version it makes it
impossible to back out of the most recent `coffee-script-source` update
to avoid the errors. This change lowers the dependency version in the
`react-rails` gemspec so that a user can specify a lower version.

Author: christopherstyles

react-rails.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'jbuilder'
 
   s.add_dependency 'execjs'
-  s.add_dependency 'coffee-script-source', '~>1.9'
+  s.add_dependency 'coffee-script-source', '~>1.8'
   s.add_dependency 'rails', '>= 3.1'
   s.add_dependency 'react-source', '~> 0.12'
   s.add_dependency 'connection_pool'