Blacklist PyQgsDBManagerGpkg test

The test consistently segfaults on Travis, but run fine locally,
normally or under Valgrind.
Trying to run it under Valgrind on Travis runners does not succeed:
https://travis-ci.org/rouault/Quantum-GIS/builds/168674938

So blacklist for now

ci/travis/linux/blacklist.txt

@@ -5,3 +5,4 @@ qgis_composermapgridtest
 qgis_composerutils
 ProcessingGrass7AlgorithmsImageryTest
 ProcessingGrass7AlgorithmsRasterTest
+PyQgsDBManagerGpkg