Travis-CI: update the list of Ruby's and Python's versions

Travis-CI no longer provides Ruby 2.0 in its Trusty environment (it has
been updated according to
https://blog.travis-ci.com/2017-06-21-trusty-updates-2017-Q2-launch).
Remove this version from .travis.yml.

Moreover Python 3.6 and Ruby 3.4 are available. Test building with them.

Last by not least, pypy virtual environment directory has been renamed
pypy2 (/home/travis/virtualenv/pypy2 is a symbolic link to pypy2-5.6.0).

The updated .travis.yml file has been tested on
https://travis-ci.org/fishilico/selinux/builds/248126824

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