Drop duplicated versions in salt.modules.pkg_resource.sort_pkglist()

This prevents having packages which report the same version multiple times such as
```
microsoft.office = ["15.0.4569.1506","15.0.4569.1506"]
```
Duplicated version will be dropped to a result like this:
```
microsoft.office = ["15.0.4569.1506"]
```
This is a result of software being registered in 2 places (registry + MSI) at the same time on Windows.
I implemented this in `salt.modules.pkg_resource.sort_pkglist()` as a general safety measure to prevent this from happening in other situations again as it lead to quite some unstable behaviour in other modules here.