Use the last version given to path command

Instead of throwing an error when two versions are given I've changed `haxelib path` to use the last version. This allows dependencies to be overridden.

For example:
`haxelib path <lib>:1.2.1 <lib>:1.2.2` returns version 1.2.2
`haxelib path <lib> <lib>:1.2.1` returns version 1.2.1

An advantage of this fix is for libraries like HaxePunk and HaxeFlixel where they can set a specific version of OpenFL instead of always using the latest.