Don't use Tox on Travis

It offers no clear advantage over running pytest directly.

.travis.yml

@@ -1,23 +1,18 @@
 sudo: false
 language: python
-matrix:
+python:
-  include:
+  - "2.7"
-    - python: 3.5
+  - "3.3"
-      env: TOXENV=py35
+  - "3.4"
-    - python: 3.6-dev
+  - "3.5"
-      env: TOXENV=py36
+  - "3.6"
-env:
+  - pypy
-  - TOXENV=py27
+install:
-  - TOXENV=py33
+  - pip install --upgrade pytest
-  - TOXENV=py34
+  - pip install -r requirements-travis.txt
-  - TOXENV=pypy
+  - pip install -e .
-  - TOXENV=flake8
+script: pytest
-install: pip install tox
+cache: pip
-script: tox
-cache:
-  directories:
-    - .tox
-    - $HOME/.cache/pip
 after_success: make coveralls
 notifications:
   email: