Mongofill compatibility

Mongofill is a pure PHP implementation of the MongoDB driver, with aim to be a drop-in replacement of the official extension, usable under HHVM runtime.

This PR fixes a couple of minor issues (choosing fields with `'fieldName' => true` vs just `field`, ensuring all `insert()` are called with a variable etc). I also used the opportunity to utilize the short array form, introduced in PHP 5.4, since this is our minimum requirement.

Tests had to be updated to not check for the Mongo extension, but instead just check for the MongoClient class.

To try this out, disable the mongo extension and run `composer require mongofill/mongofill dev-master` in the imbo installation directory, then run the tests. They all seem to pass now.

This brings Imbo one step closer to HHVM-compatibility.