Improve developer experience after cloning

:fork_and_knife:

* Show less PhantomJS logging by default
* Update cassettes to be even less dependent on a particular configuration.
* Prevent the app from being loaded unless `dotenv` can load a file.

We've discussed moving away from `dotenv`, but while we still have it, switching away from `dotenv-rails` (which uses the quiet `Dotenv.load`) in favor of `Dotenv.load!` should nip a common pitfall in the bud. Combined with the updates to the VCR cassettes, once a user has Ruby, Postgres, Node, and PhantomJS installed, they should be able to run specs.