humbug_trac: Make sure we import humbug_trac_config from cwd.

It doesn't matter for this script, but I worry the code will be
copy-pasted into other new plugins.

(imported from commit 0fe5280af5aa05a7efc3d146f1570f9a72c62027)

api/integrations/humbug_trac.py

@@ -38,6 +38,8 @@
 from trac.core import Component, implements
 from trac.ticket import ITicketChangeListener
 import sys
+import os.path
+sys.path.insert(0, os.path.dirname(__file__))
 import humbug_trac_config as config
 
 if config.HUMBUG_API_PATH is not None: