Specify Qt version in CMakeLists.txt

Summary:
In `CMakeLists.txt` Qt version don't set, so when I have tried to build `Cantor` with my `Qt 5.5`, I have had compilation errors.
So I suggest to set minimal Qt version (`5.6.0`, I have compiled with this version without errors).

Reviewers: #cantor, aacid

Reviewed By: aacid

Subscribers: #cantor, #kde_edu

Tags: #kde_edu, #cantor

Differential Revision: https://phabricator.kde.org/D11465