Use of natural sort order

I've made a tweak (with added test) so that MTMigration treats version numbers in natural sort order instead of lexicographical order. This means that version 0.10 is now treated as coming after 0.9, not the other way around.

(Yes, I included MTMigration in a project of mine, thank you, it's just what I needed! The project is at version 1.8 and I'll be hitting 1.10 soon so I can see the need for natural sort order coming! :) )