Fix all the things.

* Update `src/` to be PSR-2 compliant.
* Replace deprecated `safe` option w/ write concern.
* Remove/replace usage of deprecated `MongoClient::connected` property.
* Replace deprecated `Mongo` class w/ `MongoClient`.
* Update Travis CI config to use PHPUnit 3.7.*... (default Travis PHPUnit version has changed since last time Travis ran...)

This being said... with replacing the `safe` value from a boolean, to 0 or 1, it's a BC break. Not sure how to handle this... there's only a master branch here... heh. Pull and tag a new minor version or something? Also, let me know what you guys think / find.
Could you replace this with: `$w = $this->safe ? 1 : 0;`, this could even be inlined in the return array.