Lock Gemfile to Mongoid 3.0.13.

Fixes the problem described here:
https://github.com/mongoid/mongoid/issues/2579

This should be reverted when the Mongoid issue (or whatever it is)
is fixed.

Gemfile

@@ -2,7 +2,7 @@ source "http://rubygems.org"
 
 group :development, :test do
   gem 'rake'
-  gem 'mongoid'
+  gem 'mongoid', '3.0.13'
   gem 'bson_ext', :platforms => :ruby
 
   gem 'rails'