[Composer] Update to PSR-4 + Use Semver constraint + Remove minimum-stability

As explain in the title, this PR does three things:

 * Remove the `minimum-stability` flag as `stable` is the default one.
 * Upgrade all constraints to ones following Semver and so, should not break BC
 * Upgrade tha autoloading to PSR-4

I need this PR meanwhile we find the issues in #75 (One of my travis-ci build can't be resolved. See [here](https://travis-ci.org/egeloen/IvoryCKEditorBundle/jobs/40826801)).