EZP-2059 - Refactored composer.json

* Removed `minimum-stability` flag
* Used wildcards for versions werever possible
* Used 5.1.* branch alias for ezpublish-kernel

Still need to *copy* zeta components dependencies here because they
don't have stable tags.
Merge pull request #46 from ezsystems/composer.json-redux

EZP-2059 - Refactored composer.json