cleanup gemspec

 - remove unnecessary things
 - make gem lighter (mb) / faster to install

@brianmario
Was curious if passing the version here was compatible with earlier (but maybe still used) versions of rubygems. Looks like it was [added in Oct of 2009](https://github.com/rubygems/rubygems/commit/3efe3b056d4375fb3b2f1cd3330f47f8f9d6342c#diff-05477d38d75c98664b0e7dec6eb73b45R416) by the wise and powerful @jbarnette :)