Feature - Travis-CI improvements

Just like our [Contributing doc states][contributing-doc] states, all code in Klein should validate as valid PSR-2 code. Making it easier to review PR's, I thought it'd be a good idea to add automatic PSR-2 validation to the Travis-CI build/test process. This PR accomplishes that.

 [contributing-doc]: https://github.com/chriso/klein.php/blob/215501e6e03f000e36aeb9ebd2385168bcb4773b/CONTRIBUTING.md