Exclude PyQgsPalLabelingCanvas test because of Segfault on test server

It seems that there is still a thread running when QgsApplication exits.

Error message:
QThreadStorage: Thread 0x16e29f0 exited after QThreadStorage 6 destroyed

.travis.yml

@@ -14,4 +14,4 @@ before_script:
   - cmake -DWITH_MAPSERVER=ON -DWITH_STAGED_PLUGINS=OFF -DWITH_GRASS=OFF -DSUPPRESS_QT_WARNINGS=ON ..
   - make -j2
 
-script: xvfb-run ctest -j2 --output-on-failure -D Experimental
+script: xvfb-run ctest -j2 --output-on-failure -E PyQgsPalLabelingCanvas -D Experimental