UUID Updates

Removes 'provider' attribute from User model in favor of Authentication model for handling 3rd party authentications.
Stores third party authentications in Authentication model.
Adds generation of a UUID in :before_validation hook. 
Migration generates UUIDs and Authentication records for existing users. 
Removed 'date_of_birth' from profile placeholder hash.
Enforces capybara version in order to support 'visit' method in spec/requests (version 2+ requires tests to be in spec/features in order to use 'visit')

I made a couple of decisions in handling of the UID field and how third party authentications are handled. Please review the following:
-Opted for API to output 'uid' field as 'id' for external representation of a user's non-PII identifier (profiles_controller.rb)
-Updated User#find_for_open_id lookup query and insert (user.rb).