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:

setup.py

@@ -47,10 +47,15 @@ setup(
         datasette=datasette.cli:cli
     ''',
     setup_requires=['pytest-runner'],
+    extras_require={
+        'test': [
+            'pytest==3.6.0',
+            'aiohttp==2.3.2',
+            'beautifulsoup4==4.6.0',
+        ]
+    },
     tests_require=[
-        'pytest==3.6.0',
-        'aiohttp==2.3.2',
-        'beautifulsoup4==4.6.0',
+        'datasette[test]',
     ],
     classifiers=[
         'Development Status :: 4 - Beta',