Rely on external mongoid session store gem

This is a fix for #714. 

It involves 2 things:

First, the spec/dummy rails app was specifying `:cookie_store`. In order to test Locomotive's internal session store specification logic, I have commented this out.

That makes everything explode, so....

Second, I am relying on an external gem (https://github.com/brianhempel/mongo_session_store) for `:mongoid_store`. It has a passing test suite on TravisCI, and I now have a commit bit to it thanks to some work I did to improve it for our usage here (namely, detaching it from the Mongo gem).

This pull request should be good to go as-is. 

What remains unresolved is that, in all likelihood, NOBODY is actually using `:mongoid_store` as provided by Locomotive. Everybody is using the default `:cookie_store`, or their own `:mongoid_store` gem. (Which is why I'm the first to report this bug.) 

Something should be done about that. Some options are:
1.) Include documentation instructing people to comment out their `:cookie_store` to use our `:mongoid_store`
2.) Remove the default Locomotive `:mongoid_store` integration, and include instructions on how people can install an external gem themselves. (There are several decent options. And people might even want to use `:redis_store` or something like that if they're integrating into a larger system.)

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