fixed Qt5 compat

see scikit-image#1316
This logic is not quite right.  The reason for this `try` cascade is that when mpl added QT5 support we re-named `qt4_compat` -> `qt_compat` as it smooths over the differences between pyqt4/pysides/pyqt5 so you can not assume that if this import succeeds then you want to use qt5