Drop support for symfony/* <2.3
[1.1] Update symfony versions in fat composer.json
[1.1] Update all symfony version references in the docs to 2.3
[1.1] Add minimum-stability of dev
[1.1] Remove symfony <2.3 hacks
merged branch igorw/1.1-deps (PR #695)

This PR was merged into the master branch.

Discussion
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[1.1] Drop support for symfony/* <2.3

I've left the ranges in, because it's likely that we will extend them in the future.

Commits
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54be4f5 [1.1] Remove symfony <2.3 hacks
2b45f60 [1.1] Add minimum-stability of dev
8f8ba41 [1.1] Update all symfony version references in the docs to 2.3
7a14b20 [1.1] Update symfony versions in fat composer.json
e9478dd Drop support for symfony/* <2.3