minor #739 Remove unused twig-extensions dependency (tommygnr)

This PR was submitted for the 2.3 branch but it was merged into the 2.7 branch instead (closes #739).

Discussion
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Remove unused twig-extensions dependency

The ```twig/extensions``` library is not actually used or enabled in the symfony standard distribution so its inclusion in the composer.json file seems unnecessary. I did quite a bit of digging around to work out why it was ever included and as best I can tell it was because in the betas of ```2.0``` there was a convenient way to register these extensions in the twig configuration block. That approach was abandoned in symfony/symfony@9604573

Commits
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1272c9f Remove unused twig-extensions dependency

Author: fabpot

composer.json

@@ -12,7 +12,6 @@
         "doctrine/orm": "~2.2,>=2.2.3,<2.5",
         "doctrine/dbal": "<2.5",
         "doctrine/doctrine-bundle": "~1.4",
-        "twig/extensions": "~1.0",
         "symfony/assetic-bundle": "~2.3",
         "symfony/swiftmailer-bundle": "~2.3",
         "symfony/monolog-bundle": "~2.4",