bump to 1.5.9

Can you release a new version of the gem please.
This change in https://github.com/drnic/rubigen/pull/11 never made it to v1.5.8 but is needed to work with rubygems 2x. (we get undefined method `source_index' for Gem:Module and we need this https://github.com/jarib/rubigen/commit/dc00d3616a34bc72728df7d9adbb4af69d2affeb)

Thanks so much.