Candidate for 2.0.2 release updating rubyzip to >1.2.1

[tomas...@showmax.com]

Please consider using my branch https://github.com/graywolf-at-work/axlsx/tree/update_rubyzip_for_2.0_series for making 2.0.2 release to update rubyzip.

I've also switched to use minitest instead of test/unit and removed dead ruby versions from travis (and added new alive ones).

Please consider for release.