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.
````
load only VERSION in the gemspec

previously loaded schiphol which loads `zip/zip` which may not be
available at gemspec time
duplicate the VERSION object so it can be stripped

should fix:
````
/Users/turadg/.rbenv/versions/1.9.3-p327-perf/lib/ruby/1.9.1/rubygems/ve
rsion.rb:191:in `strip!': can't modify frozen String (RuntimeError)
	from
/Users/turadg/.rbenv/versions/1.9.3-p327-perf/lib/ruby/1.9.1/rubygems/ve
rsion.rb:191:in `initialize'
	from
/Users/turadg/.rbenv/versions/1.9.3-p327-perf/lib/ruby/1.9.1/rubygems/ve
rsion.rb:178:in `new'
	from
/Users/turadg/.rbenv/versions/1.9.3-p327-perf/lib/ruby/1.9.1/rubygems/ve
rsion.rb:178:in `create'
	from
/Users/turadg/.rbenv/versions/1.9.3-p327-perf/lib/ruby/1.9.1/rubygems/sp
ecification.rb:2129:in `version='
	from
/Users/turadg/Code/Ruby/geknowm-rails/vendor/bundle/ruby/1.9.1/bundler/g
ems/schiphol-dce8ec894aaf/schiphol.gemspec:7:in `block in <main>'
````
refactor class into a module
Merge pull request #4 from turadg/master

Refactor class into a module