Handle binary downloads

Also a more reliable fix for #2
include `rubyzip` in Gemfile

http://gembundler.com/rubygems.html

to avoid this error:
````
       There was a LoadError while evaluating schiphol.gemspec:
       no such file to load -- zip/zip from

/tmp/build_26b8yso2vcet4/vendor/bundle/ruby/1.9.1/bundler/gems/schiphol-
553c5439bfda/schiphol.gemspec:3:in `<main>'
       Does it try to require a relative path? That's been removed in
Ruby 1.9.
````