Move Coverage reporting to latest env

[Koopzington]

Requiring php-coveralls results in a full update of all dependencies instead of composer installing the versions mentioned in the composer.lock

[marc-mabe]

@Koopzington This will slow down the tests a lot as generating the code coverage and looking up the latest versions both takes additional time and should be spitted over different build jobs

[Koopzington]

uhm... what would slow the tests down? It's not like i added some new feature, i merely moved the execution to the environment where it's supposed to be.
Build execution times haven't increased either compared to the one before this one:
https://travis-ci.org/zendframework/zend-cache/builds/159688399
https://travis-ci.org/zendframework/zend-cache/builds/152799417

[marc-mabe]

@Koopzington Oh you are right - installing the latest dependencies only takes some seconds more than installing from lock. the biggest overhead comes from code coverage (~2mins more) but we have to do it at least once