Bump version to v0.3.0 in version.rb

This was missed when updating the CHANGELOG in #27, and appears to be
the cause of #28.

I'm not 100% this is the only thing that needs to be done to trigger a
publication of the v0.3.0 Gem, let me know if there's anything else that
needs to be done.
Merge pull request #29 from mikepea/bump_v0_3_0

Bump version to v0.3.0 in version.rb