Raising the requirement to Qt 5.5 ?

[David Faure]

We decided some time ago to support the last three minor Qt 5 release.

Qt 5.7 was released on 16th June 2016, only 3 months after Qt 5.6 was released 
on 16th March 2016 (and 1 year after 5.5), but if we apply the above rule, it 
would mean we can drop Qt 5.4 support and require Qt 5.5.
Of course at the time of making up the rule we didn't expect two minor 
releases only 3 months apart...

This is being prompted by https://git.reviewboard.kde.org/r/128232 which would 
fix an inconsistency in kdebugsettings, so nothing critical for sure, it could 
wait.

Thoughts ?

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE Frameworks 5