Extending deppendency and php support

- Even tho the parallel extension requires PHP 7.2, the single thread execution should be able to support PHP 5.6, 7.0 and 7.1 too, thus the travis tests should cover these environment setups too. From 7.2 the features should be tested with and without parallel too.
- There is point in requiring a fresh composer version for the installer, then old versions can do the trick too.
