add phpunit to dev-requirements

Changed require_once call to require; since phpunit autoloads composer, the require_once call was returning `true` as in _this file has already been required_.
I guess there's 2 ways of doing this:

1. You require phpunit using composer, phpunit will then load the autoloader, and travis should use the phpunit in vendor/bin
2. You depend on devs installing phpunit globally, in which case I'll fix this up and add a note to the readme for testing

@asm89 I'm guessing you guys would rather go for option 2, so I'll make that change now. If people don't like it, it's easy enough to change in the future.