Declare an activemodel dependency

mocks.rb requires active_model, therefore it should
be listed in the gemspec.

Author: jfirebaugh

Changelog.md

@@ -17,6 +17,8 @@ Bug fixes
 * Where possible, check against the version of ActiveRecord, rather than
   Rails. It is possible to use some of rspec-rails without all of Rails.
   (Darryl Pogue)
+* Explicitly depends on `activemodel`. This allows libraries that do not bring
+  in all of `rails` to use `rspec-rails`. (John Firebaugh)
 
 ### 2.14.0 / 2013-07-06
 [full changelog](http://github.com/rspec/rspec-rails/compare/v2.14.0.rc1...v2.14.0)

rspec-rails.gemspec

@@ -22,6 +22,7 @@ Gem::Specification.new do |s|
   s.require_path     = "lib"
 
   s.add_runtime_dependency(%q<activesupport>, [">= 3.0"])
+  s.add_runtime_dependency(%q<activemodel>, [">= 3.0"])
   s.add_runtime_dependency(%q<actionpack>, [">= 3.0"])
   s.add_runtime_dependency(%q<railties>, [">= 3.0"])
   %w[core expectations mocks].each do |name|