Update library configuration

### Git
* added the `composer.lock` to `.gitignore`
### Composer
* added  `codesniffer` with version 2.0.*@dev (while waiting for 2.0 stable) to composer
* update `PHPUnit` to minimum version 4.0
* removed `coverall`
### Travis CI
* moved `coverall.io` config from composer to Travis CI configuration file
* added `PHP 5.6` and `HHVM` to Travis CI configuration file
### PHPUnit
* added code coverage to `PHPUnit` config file