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
Merge branch 'rubyzip_fix' of github.com:PragTob/warbler into PragTob-rubyzip_fix
added a comment to the Gemfile to explain why the warning messages are generated when using the gemspec directive
resolved some duplication between the Gemfile and the gemspec