Update GMT::Bsmap::MethRatio

Supersedes #244.

- [ ] Pull test data from Bsmap
- [ ] Update tests to run `methratio.py`
- [ ] Verify that removing default version doesn't break anything
It should work!

Since the ordering of `keys` is random, maybe `available_methratio_versions` should sort?  It is a little weird if the documentation varies each time it runs.