[DCOM-96] proxy generation as of doctrine/common#168

See doctrine/common#168, doctrine/doctrine2#406 and doctrine/mongodb-odm#445

Successful build at https://travis-ci.org/Ocramius/phpcr-odm/builds/3156953 (modified composer.json, not included in this PR)

Merging this means bumping requirement of `doctrine/common` to `>=2.4`.