Speed up Travis by reusing pip wheel cache across builds (#324)

* Cache pip wheels between runs in Travis, refs #323
* Run pytest manually - "python setup.py test" appeared to still download a bunch of stuff: https://travis-ci.org/simonw/datasette/jobs/395306188

* Use extras_require so pip can install test dependencies: https://github.com/pypa/pip/issues/1197#issuecomment-228939212

.travis.yml

@@ -7,7 +7,13 @@ python:
 
 # Executed for 3.5 AND 3.5 as the first "test" stage:
 script:
-  - python setup.py test
+  - pip install -U pip wheel
+  - pip install .[test]
+  - pytest
+
+cache:
+    directories:
+        - $HOME/.cache/pip
 
 # This defines further stages that execute after the tests
 jobs: