Temporarirly remove pypy3 from CI build matrix

Currently Travis has an issue with pypy3 builds, see:

travis-ci/travis-ci#6727

Once this has been fixed on travis we can restart testing against it.
Please note that 'tox -e pypy3' still works locally, so your PRs will
still be checked against pypy3 before a merge (so make sure it passes).

.travis.yml

@@ -15,7 +15,7 @@ env:
   - TOX_ENV=py34
   - TOX_ENV=py35
   - TOX_ENV=pypy
-  - TOX_ENV=pypy3
+    #- TOX_ENV=pypy3
   - TOX_ENV=flake8
 install: travis_retry pip install tox
 script: