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 develop -D org -I include -E exclude: PR 528 (Tile performance improvements for VMS and NDPI)
merge develop -D org -I include -E exclude: PR 834 (Permit cppwrap failure)
merge develop -D org -I include -E exclude: PR 847 (Fix the copyright statement on generated Javadocs (rebased onto develop))
merge develop -D org -I include -E exclude: PR 848 (xsd-fu: Further work for C++ metadata store generation)
merge develop -D org -I include -E exclude: PR 849 (travis: Remove python version requirement)