[docs] Adjust symfony/* constraints to use tilde operator

This will hopefully make upgrade to 2.2 more seamless
merged branch igorw/constraints-docs (PR #616)

This PR was merged into the master branch.

Commits
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64b66b1 [docs] Adjust symfony/* constraints to use tilde operator

Discussion
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[docs] Adjust symfony/* constraints to use tilde operator

This will hopefully make upgrade to 2.2 more seamless