Simplified the Travis build matrix

Testing the older releases and the dev versions of vendors separately and against all PHP versions is wasting the Travis resources and making builds longer given that the number of concurrent jobs is limited (5 per repo owner currently AFAIK).
Testing these libraries on each PHP version is not needed (they already do it) and they can be combined in the same build.
This only going down from 20 jobs per build to 6.

I also fixed a wrong assertion in the testsuite (I have no idea why PHPUnit 3.7 was not failing when running it, but PHPUnit 4 detected the difference)