Install dotenv gem to store sensitive keys

Sensitive keys (like Janrain credentials) can be committed to public
version control.  And 12factor.net/config suggest extracting all
environment-specific configs to environment variables.

Setup dotenv to load them from the .env file.
Add janrain_uuid to User model

Most applications will integrate with Janrain by recording a user's
Janrain UUID.

Record ours in the users table.