Fixed unrar version check.
Review Request #129255 - Created Oct. 24, 2016 and submitted
The parameter "--version" stopped working since unrar 5.2 which was
released almost 2 years ago.
With this change this now becomes the minimum version required but
every distribution that is going to include a version of KDE with
this commit has probably been using unrar > 5.2 for quite a while
so I don't think there is any need to provide a fallback.
This fixes bugs 367626 and 341305
Verified that the previously missing thumbnails for cbr files are now being correctly generated.