Install optional dependency urwid in Travis.

This is to fix some pylint errors. Pylint couldn't reason about the
urwid classes because they were not installed.

.travis.yml

@@ -6,6 +6,7 @@ python:
   - "3.4"
 install:
   - "pip install ."
+  - "pip install urwid"
   - "pip install icalendar"
   - "pip install pylint"
 script: "./run-tests.sh"