Remove EOLed Symfony versions from Travis

Currently, the testsuite relies on SensioFrameworkExtraBundle 3.x which requires Symfony 2.3+. Given that 2.1 and 2.2 are EOLed, removing them from Travis is simpler than updating the testsuite to support them.
Merge pull request #514 from stof/patch-2

Remove EOLed Symfony versions from Travis