Adding symfony/expression-language to require-dev

I wanted to fix the tests so they pass. Another user already has a pull request doing exactly this, but he was unable to rebase his changes properly.

I hope this is the right thing to do.
Merge pull request #432 from inssein/patch-2

Adding symfony/expression-language to require-dev
Update VoteInterface.php
Update isVoteValid so that it uses the new ExecutionContextInterface
Support 2.3 LTS
Update VoteInterface.php

Support 2.3 LTS