Remove mongoose-bluebird-utils

This removes `bluebird-utils` as a dependency and adds a pretty warning
message for when someone tries to use a ref-promise to a Model which
isn't "promisified". It also updates the dependencies and adds code
coverage enforcement.

We'll have to add this notice to the README.
Set-up coveralls

This sets-up travis to upload code coverage metrics to coveralls.io and
adds the `lcov` file (generated by `mocha-spec-cov-alt`) to the
`.gitignore`.
Find a better, non-breaking solution

This "promisifies" `mongoose.Model.find` and `mongoose.Model.findById`
and uses the functions with `.call` rather than promisifying the whole
model on the method. No changes need to be made to the README.