Add Rails::Observers dependency

Extracted from Rails core.

Signed-off-by: Alex Coles <alex@alexbcoles.com>

Gemfile

@@ -47,6 +47,7 @@ source 'https://rubygems.org'
 gem "rails", "= 4.0.9"
 gem 'protected_attributes'
 gem 'actionpack-action_caching'
+gem 'rails-observers'
 
 gem "coderay", "~> 1.0.5"
 gem "rubytree", "~> 0.8.3"

Gemfile.lock

@@ -256,6 +256,8 @@ GEM
       bundler (>= 1.3.0, < 2.0)
       railties (= 4.0.9)
       sprockets-rails (~> 2.0)
+    rails-observers (0.1.2)
+      activemodel (~> 4.0)
     rails_autolink (1.1.6)
       rails (> 3.1)
     railties (4.0.9)
@@ -437,6 +439,7 @@ DEPENDENCIES
   rack-test (~> 0.6.2)
   rack_session_access
   rails (= 4.0.9)
+  rails-observers
   rails_autolink
   rb-readline (~> 0.5.1)
   rdoc (>= 2.4.2)