Move dependencies to the gemspec and lock down versions.

Development dependencies should go in the gemspec when using a Gemfile. Versions
are locked down to prevent failures creeping in as dependencies are updated (so
long as they play nice with semantic versioning). An example of this is the
minitest gem; see issue 164 (https://github.com/rubyzip/rubyzip/issues/164).
Update tests to use the minitest 5.x API.

MiniTest::Unit::TestCase -> MiniTest::Test
MiniTest::Unit.after_tests -> MiniTest.after_run