Use the latest version of minitest.

Now the real culprit of the random test failures has been fixed [1] we can use
the latest version of minitest.

[1] https://github.com/amco/rubyzip/commit/96f84aee4c575a0c886462abb809ef47e2866f94
Merge pull request #174 from hainesr/latest-minitest

Use the latest version of minitest.