SymfonyRequirements.php: add filter requirement
minor #122 SymfonyRequirements.php: add filter requirement (glensc)

This PR was squashed before being merged into the 3.0.x-dev branch (closes #122).

Discussion
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SymfonyRequirements.php: add filter requirement

filter extension should be enabled, or you may get such error when validating:

UndefinedFunctionException: Attempted to call function "filter_var" from namespace "Symfony\Component\Validator\Constraints"

Commits
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f2b8d1d SymfonyRequirements.php: add filter requirement