fixing build - step 1 - adjusting composer.json

Since [87f2072](https://github.com/sonata-project/SonataUserBundle/commit/87f2072) we require PageableManagerInterface, but the interface was added in core-bundle [4f48318](https://github.com/sonata-project/SonataCoreBundle/commit/4f48318), which is beyond what we get with 
>"sonata-project/core-bundle": "~2.2"

(composer will checkout 2.2.5)
So to green the build, we need to checkout dev version :/