Update GMT::Bsmap::MethRatio

Supersedes #244.

Before merging I would like to update the tests so they actually run `methratio.py`.
Personal preference, but based on the doc string for `reference` do you think this should validate that the fasta exists at the given path if it doesn't find it in the hash? That way we can die with a good error message rather than passing a bad value to `-d` below.