Don't pass version to tox explicitly

Author: untitaker

.travis.yml

@@ -23,11 +23,10 @@ matrix:
 
 
 install:
-    - pip install tox>=1.8
+    - pip install tox
 
 script:
-    - tox -e \
-      $(echo py$TRAVIS_PYTHON_VERSION | tr -d . | sed -e 's/pypypy/pypy/')-$REQUIREMENTS
+    - tox -e py-$REQUIREMENTS
 
 branches:
   except: