travis: Remove python version requirement

Travis recently changed how python is supported, but apparently
what we were doing never actually worked: if you specify java
as the language, the python block is ignored.  So we're stuck
with whatever the build environment for the specific java
version provides.  This patch removes python from the matrix
excludes, which should restore the matrix exclusions.
Merge pull request #850 from rleigh-dundee/travis-python-dev50

travis: Remove python version requirement