One Travis instance should run minimum supported QT version #14983
Description
@jonasschnelli wrote in #14979:
#14573 broke < Qt5.6 compatibility due to calling the lambda version of
addActionthat was added in Qt5.6.
We currently support QT >=5.2, but hardly anyone tests with that. If we make one of the Travis machines use it, we should have a better change of catching this.
See also discussion about minimum QT version in #13478