Allow ZF2.3 to be installed

The ~x.y notation allows any version of x where x.n >= x.y. This means that previously only 2.1.x and 2.2.x were allowed, not all new versions (like 2.3.x and so on) can be used as well.
Correct composer dependencies to allow 2.x
Merge pull request #53 from juriansluiman/feature/update-composer-requirements

Allow ZF2.3 to be installed