update Travis build matrix

I thought that it is not really necessary to run tests against all available Symfony versions for every PHP version. Running them only with PHP 5.6 should be sufficient.

As far as I can see, there is also no reason to change the version constraints, but just improve the checks if the ExpressionLanguage support is available for the DependencyInjection component.

This needs to be rebased when #22 is merged.