Lower the coffee-script-source version specifier

There is an issue with the latest version of `coffee-script-source`
outlined in jashkenas/coffeescript#382.

The `react-rails` gemspec specifies a `~>1.9` version of
`coffee-script-source`, which makes it impossible to back out of
its most recent version in order to avoid the errors (for example,
if you want to specify `~>1.8` in your app).

This change lowers the dependency version in the `react-rails`
gemspec so that a user can specify a lower working version.

Author: sportsman701

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'