Added support for travis-ci and a first spec

As seen https://travis-ci.org/ngiger/ydocx/builds/19102146 the spec run on
* ruby head
* ruby 2.0
* Ruby 1.9.3
* Ruby 1.9.2
Reverted to use rubyzip 0.9.9. This is the version actually in use at oddb.org.

Documented requirements for Ruby >= 1.9.2 and rubyzip <1.0.0.