allow failures on nightly Python

while it's useful to run the tests,
failures on Python nightly are likely to be not our bugs,
and shouldn't generally be considered CI failures.

.travis.yml

@@ -1,22 +1,26 b''
 # http://travis-ci.org/#!/ipython/ipython
 language: python
 python:
     - "nightly"
     - 3.5
     - 3.4
     - 3.3
     - 2.7
 sudo: false
 before_install:
     - git clone --quiet --depth 1 https://github.com/minrk/travis-wheels travis-wheels
     - 'if [[ $GROUP != js* ]]; then COVERAGE=""; fi'
 install:
     - pip install "setuptools>=18.5"
     - pip install -f travis-wheels/wheelhouse -e file://$PWD#egg=ipython[test]
     - pip install codecov
 script:
     - cd /tmp && iptest --coverage xml && cd -
 after_success:
     - cp /tmp/ipy_coverage.xml ./
     - cp /tmp/.coverage ./
     - codecov
+
+matrix:
+    allow_failures:
+        python: nightly