Adjust to rubyzip 1.0 release (e.g. corrected incompatibilities)

* had to adjust one spec that relied on internal rubyzip structure
  that changed
* removed duplicated gem dependencies from Gemfile to use gemspec,
  development gem group probably should also be defined there
* Addresses #190

Turns out I did have a little bit of time and the fix seems easy :-)

Cheers and thanks for warbler,
Tobi