Make Travis run the tests on python 3

The python 3 tests are allowed to fail so they will not mark pull
requests as broken.

.travis.yml

@@ -1,17 +1,21 @@
-env:
-   global:
-      - secure: "HIj3p+p2PV8DBVg/KGUx6n83KwB0ASE5FwOn0SMB9zxnzAqe8sapwdBQdMdq0sXB7xT1spJqRxuxOMVEVn35BNLu7bxMLfa4287C8YXcomnvmv9xruxAsjsIewnNQ80vtPVbQddBPxa4jKbqgPby5QhhAP8KANAqYe44pIV70fY="
-      - GH_DOC_BRANCH: develop
-      - GH_REPOSITORY: github.com/MDAnalysis/mdanalysis.git
-      - GIT_CI_USER: TravisCI
-      - GIT_CI_EMAIL: TravisCI@mdanalysis.org
-      - MDA_DOCDIR: package/doc/html
-   matrix:
-   - SETUP=full CYTHONIZE=true
-   - SETUP=minimal CYTHONIZE=false
 language: python
-python:
-  - "2.7"
+env:
+  global:
+    - secure: "HIj3p+p2PV8DBVg/KGUx6n83KwB0ASE5FwOn0SMB9zxnzAqe8sapwdBQdMdq0sXB7xT1spJqRxuxOMVEVn35BNLu7bxMLfa4287C8YXcomnvmv9xruxAsjsIewnNQ80vtPVbQddBPxa4jKbqgPby5QhhAP8KANAqYe44pIV70fY="
+    - GH_DOC_BRANCH: develop
+    - GH_REPOSITORY: github.com/MDAnalysis/mdanalysis.git
+    - GIT_CI_USER: TravisCI
+    - GIT_CI_EMAIL: TravisCI@mdanalysis.org
+    - MDA_DOCDIR: package/doc/html
+matrix:
+  allow_failures:
+     - python: "3.3"
+       env: SETUP=full CYTHONIZE=true
+  include:
+     - python: "2.7"
+       env: SETUP=minimal CYTHONIZE=false
+     - python: "2.7"
+       env: SETUP=full CYTHONIZE=true
 # command to install dependencies
 addons:
   apt: