bug#1194 Make Twig HHVM compatible (fabpot)

This PR was merged into the master branch.

Discussion
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Make Twig HHVM compatible

Work in progress

Commits
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7a7e8c5 removed a test that do not pass on hhvm as it is not able to compare DateTime objects properly
89779bc fixed a test on hhvm
f939ea6 skipped a test on hhvm
cbbcd89 fixed some tests as hhvm always returns false for ReflectionArgument::isOptional()