Drop the PURE_PYTHON travis builds, they're not doing anything.

We publish manylinux wheels now and pip will install those. (The same
is probably true for tox, but the end user may be able to control that
better so I'm leaving those environments for now.)

We test PyPy so we do cover the pure environments.

.travis.yml

@@ -2,15 +2,11 @@ language: python
 sudo: false
 matrix:
   include:
-    - python: 2.7
-      env: PURE_PYTHON=1
     - python: 2.7
       env: DOCS=1
       # The doctests only run on Python 2.7
     - python: 2.7
       env: MINIMAL="-t !persistentregistry -t !security"
-    - python: 3.5
-      env: PURE_PYTHON=1
 python:
   - 2.7
   - 3.4
@@ -22,7 +18,7 @@ python:
 script:
   - coverage run -m zope.testrunner --test-path=src $MINIMAL
   - if [[ -n "$DOCS" ]]; then sphinx-build -b html    -d docs/_build/doctrees docs docs/_build/html; fi
-  - if [[ -n "$DOCS" ]]; then sphinx-build -b doctest -d docs/_build/doctrees docs docs/_build/doctest; fi
+  - if [[ -n "$DOCS" ]]; then coverage run `which sphinx-build` -b doctest -d docs/_build/doctrees docs docs/_build/doctest; fi
 
 after_success:
   - coveralls