Point carrierwave-mongoid and mongoid-grid_fs to master branches to reso...

...lve activemodel dependency error

When running `bundle install` I got the following error:

````
Bundler could not find compatible versions for gem "activemodel":
  In Gemfile:
    carrierwave-mongoid (>= 0.4) ruby depends on
      activemodel (~> 3.1) ruby

    rails_admin (>= 0) ruby depends on
      activemodel (4.0.0.beta1)
````

I did some research and figured out the solution was to point carrierwave-mongoid and mongoid-grid_fs to their master branches. carrierwave-mongoid has a recent update to support Rails 4, but it requires explicitly adding the mongoid-grid_fs dependency in the rails_admin Gemfile.

I'm not sure if anyone else is able to run `bundle install` without this fix on the rails 4 branch, but I wasn't able to.