Version list presented in suboptimal ordering

The version list (i.e. that produced by `rgbenv available [--online]`) is sorted in a suboptimal way that can "hide" newer versions.
There are two problems:
- The suffix / RC versions. RC releases should come before the non-RC one with same version number.
- currently the RCs appear later than the final version e.g. `0.9.0-rc1` follows `0.9.0`
- The three numeric fields `A.B.C` aren't treated like numbers, which will be an issue wherever the number of characters differs.
- e.g. (actually it's just this one) `0.3.10` appears between `0.3.1` and `0.3.2`

Sorting using the version tags should be doable, with a bit of fiddling around anyway!