update build matrix for rails 4.0.0.beta1 and ruby 2.0.0

@justinfrench This updates for the rails 4 beta and testing with ruby 2.0.0.  While updating I had to put a RUBY_VERSION condition into the gemspec for rcov, since it won't build under ruby 2.0.0.  That being said, rcov doesnt actually work in the 1.9.x series either (although it will install).  Do you want to just completely remove rcov?