This switches the travis environment to Ubuntu Xenial 16.04, which was a long standing goal.
- It comes with a more recent version of gcc, which enables to build some dependencies, such as qt5.9 (for windows cross-builds) and rapidcheck (fails to compile at all on gcc4.8)
- It comes with a more recent version of python3, which migh fix some spurious failures in the functional tests. See #11777