minor symfony#77 Updated the application to Symfony 2.7.1 (javiereguiluz)

This PR was merged into the master branch.

Discussion
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Updated the application to Symfony 2.7.1

After discussing about the update policy in symfony#76, it's been decided to keep the Symfony Demo application in sync with Symfony's stable versions. This PR updates the application to 2.7.1 version.

Commits
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560075e Updated the application to Symfony 2.7.1

Author: javiereguiluz