Travis-CI: upgrade PyPy to 6.0

PyPy 6.0 was released in April 2018 but became available on Travis-CI
only recently. Now that it is available, use it.

While at it, compile Python modules with libpypy-c.so, now that PyPy
provides this library. This would enable linking Python modules with
-Wl,-no-undefined (or -Wl,-z,defs) and help fixing issues with
https://github.com/SELinuxProject/selinux/pull/130.

Tracking issues:
* https://github.com/travis-ci/travis-ci/issues/9542
* https://travis-ci.community/t/pypy-2-7-on-xenial/889

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>
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