Fix RemovedInPytest4Warning with dbdiff marks

Fixes yourlabs#14.
Closes yourlabs#13.

Author: blueyed

dbdiff/plugin.py

@@ -14,7 +14,7 @@ def your_test():
 
 @pytest.fixture(autouse=True)
 def _dbdiff_marker(request):
-    marker = request.keywords.get('dbdiff', None)
+    marker = request.node.get_closest_marker('dbdiff')
     if not marker:
         return