Added activemodel as a explicit dependency

As we have other dependency explicitly here
it's fine to add activemodel as well

rails.gemspec

@@ -21,6 +21,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'activesupport', version
   s.add_dependency 'actionpack',    version
   s.add_dependency 'actionview',    version
+  s.add_dependency 'activemodel',   version
   s.add_dependency 'activerecord',  version
   s.add_dependency 'actionmailer',  version
   s.add_dependency 'railties',      version