Remove hard-coded release date from gemspec

The release date in the gemspec has not been updated since version 0.16.0. As a result, the release date for the last 4 versions appears as September 26, 2012 at http://rubygems.org/gems/cassandra/versions. Specifying a release date is not necessary. If it is missing from the gemspec, it will be set automatically by the rubygems.org server.