Use tox within travis

This deduplicates the testing setup and commands

.travis.yml

@@ -6,11 +6,9 @@ python:
   - 3.5
 
 install:
-  - pip install -e .
+  - pip install tox
-  - pip install flake8
 
 script:
-  - py.test -s
+  - tox -e py
-  - flake8 radicale
 
 sudo: false