travis: add pydbus mockup

Unfortunately, python3-gi is unavailable in travis-ci environment (and
installing it from repository doesn't help, because system python is not
used). Instead, add mockup of pydbus module - to silence ImportError.
Tests will cover it with unittest.mock anyway.

test-packages/pydbus.py

@@ -0,0 +1,2 @@
+class SystemBus(object):
+    pass