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.
The tests against specific Symfony versions are now running only for a single PHP version to limit the number of jobs in the build matrix. They are also installing the full Symfony repo to be sure that all components are actually at the specified version without the need to require each of them explicitly.