Re-enable Python 3.5 testing on Travis

This is PR #875, but reissued against the pytest-2.7 branch.

According to the docs, it looks like this should work:
http://docs.travis-ci.com/user/languages/python/

I've explicitly specified beta 3, so it will need updating when future
betas come out. You could instead specify '3.5-dev', which would give
less need to update, but then it's less clear whether a change in status
is due to a change in pytest or in Python itself.

.travis.yml

@@ -1,7 +1,7 @@
 sudo: false
 language: python
 python:
-    - 'nightly'
+    - '3.5.0b3'
 # command to install dependencies
 install: "pip install -U tox"
 # # command to run tests
@@ -28,10 +28,6 @@ env:
     - TESTENV=doctesting
     - TESTENV=py27-cxfreeze
     - TESTENV=coveralls
-matrix:
-  allow_failures:
-    # py35 is currently broken on travis, see #744
-    - env: TESTENV=py35
 script: tox --recreate -i ALL=https://devpi.net/hpk/dev/ -e $TESTENV
 
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