Improve constraints of fat composer.json

* Change symfony/* from 2.1.* => ~2.1
* Delegate twig and monolog version selection to bridges
merged branch igorw/fat-constraints (PR #615)

This PR was merged into the master branch.

Commits
-------

7c1f6a5 Improve constraints of fat composer.json

Discussion
----------

Improve constraints of fat composer.json

* Change symfony/* from 2.1.* => ~2.1
* Delegate twig and monolog version selection to bridges
Merge remote-tracking branch 'fabpot/master' into verbose-and-multi-exception-logging
Updated doc with info about new parameter