Install pylint to make pypy3 runs succeed on Travis CI

pypy was still broken in the Graph class, but in this branch this is no longer an issue.

.travis.yml

@@ -9,12 +9,12 @@ python:
 matrix:
   allow_failures:
     - python: "3.2"
-    - python: "pypy3"
 install:
   - "python -m pip install pip --upgrade"
   - "pip install ."
   - "pip install .[ical]"
   - "pip install .[test]"
+  - "pip install pylint"
   - "pip install codecov"
 script:
   - "green -vvr"