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
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7c1f6a5 Improve constraints of fat composer.json

Discussion
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Improve constraints of fat composer.json

* Change symfony/* from 2.1.* => ~2.1
* Delegate twig and monolog version selection to bridges