Install prompt-toolkit to fix pylint errors in Travis CI build

.travis.yml

@@ -9,6 +9,7 @@ install:
   - "python -m pip install pip --upgrade"
   - "pip install ."
   - "pip install .[ical]"
+  - "pip install .[prompt-toolkit]"
   - "pip install .[test]"
   - "pip install pylint"
   - "pip install codecov"