Sort the list of available versions in 'pkg.list_available()'

Improves readability of `pkg.list_available`'s output. Previously it was hard to determine the latest available version because the list was not sorted. Now it is sorted ascending.

**Before:**
```YAML
MN1221-C0007:
    - 28.0
    - 25.0
    - 24.0
    - 29.0
    - 27.0.1
```
**After:**
```YAML
MN1221-C0007:
    - 24.0
    - 25.0
    - 27.0.1
    - 28.0
    - 29.0
```