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
Clean up local_entry_test.rb generated files.

Now force file generation into the test/data/generated directory where other
such files go so tests are cleaned up properly.
Clean up deflater_test.rb generated files.

Now force file generation into the test/data/generated directory where other
such files go so tests are cleaned up properly.
Clean up emptyOutDir generated file.

Now force file generation into the test/data/generated directory where other
such files go so tests are cleaned up properly.
Clean up okToDeleteMoved.txt generated file.

Now force file generation into the test/data/generated directory where other
such files go so tests are cleaned up properly.
Clean up output_stream_test.rb generated files.

Now force file generation into the test/data/generated directory where other
such files go so tests are cleaned up properly.
Clean up and disambiguate extEntry generated file.

Now force file generation into the test/data/generated directory where other
such files go so tests are cleaned up properly.
Clean up test/dummy.txt generated file.

Now force file generation into the test/data/generated directory where other
such files go so tests are cleaned up properly.
Clean up .gitignore file.

Loads of stuff in there that no longer needs ignoring.
Don't use deprecated File.exists? in tests.
Remove unused variable 'comment' in file_test.rb.
Fix #171