Bug in os relase version comparison for OSX user and group modules

This fixes a bug in the OSX user/group modules where it uses a helper function `_osmajor` to get a floating representation of the OS release version. The problem with `_osmajor` is that it parses the release version as a float which does not correctly handle versions `a.b` where `b>=10` so version 10.10 will then look like 10.1.

In fact, this bug is now evident with the release of OSX Yosemite which is version 10.10.

A better approach is to use the already defined grain `osrelease_info` which provides a tuple and then compare to a tuple of the version to test. The tuple comparison does the right thing - comparing the first number, then the seconds, etc. (https://docs.python.org/2/tutorial/datastructures.html#comparing-sequences-and-other-types)