Install from wheel when testing in travis

This helps us catch issues with MANIFEST.in and friends that
might cause problems with files not being included in the final
built package, removing a step from the release process.

.travis.yml

@@ -5,8 +5,9 @@ services:
 python:
 - 3.4
 install:
-- pip install -e .
-- pip install pyyaml
+- pip install --no-cache-dir pyyaml pytest wheel
+- python setup.py bdist_wheel
+- pip install dist/*.whl
 
 script:
   - pytest -s -v tests/${REPO_TYPE}