Use tox instead of detox (carltongibson#996)

Remove detox, since it's an unnecessary complicating factor. The
majority of the build time is container creation in Travis. Adding tox
run parallelization only adds nominal benefits.

.travis.yml

@@ -11,12 +11,11 @@ python:
 cache: pip
 
 install:
-  # latest detox and tox aren't compatible https://github.com/carltongibson/django-filter/issues/990
-  - pip install coverage detox==0.13 tox~=3.2.1 tox-travis tox-venv
+  - pip install coverage tox tox-travis tox-venv
 
 script:
   - coverage erase
-  - detox
+  - tox
 
 matrix:
   fast_finish: true