travis: disable -Werror on clang and Qt4

Otherwise, the build fails with the following error message:

/usr/include/qt4/QtCore/qsharedpointer_impl.h:595:43: error: field 'd' is
      uninitialized when used here [-Werror,-Wuninitialized]
    inline QWeakPointer(X *ptr) : d(ptr ? d->getAndRef(ptr) : 0), value(ptr)
                                          ^