Add missed dependency weakrefmethod

This probably needs to be fixed in signalslot actually, but for now,
this will do.

.travis.yml

@@ -10,6 +10,7 @@ install:
   - pip install pybuilder
   - pip install coveralls
   - pip install signalslot
+  - pip install weakrefmethod
   - pyb install_dependencies
 script: "pyb verify analyze"
 after_success: