removed deps checks in unit tests

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

As Composer is now widely used in the PHP world, having to run composer install before running the test suite is expected. This also has the nice benefit of removing a bunch of code, making things easier to maintain (there is only one place to declare a dev dependency), and probably more.

see fabpot/Silex#626 where we did the same a while ago for Silex.