TST: Pin virtualenv used on Travis CI. [skip appveyor]

numpy · jaimefrio · Jan 10, 2016 · Jan 3, 2016 · Jan 11, 2016 · Jan 9, 2016
commit a79c2da4abbf73a841d59e2cff8f93022285243a
diff --git a/.travis.yml b/.travis.yml
@@ -77,6 +77,10 @@ before_install:
   - pushd builds
   # Build into own virtualenv
   # We therefore control our own environment, avoid travis' numpy
+  #
+  # Some change in virtualenv 14.0.5 caused `test_f2py` to fail. So, we have
+  # pinned `virtualenv` to the last known working version to avoid this failure.
+  - pip install -U 'virtualenv==14.0.4'
   - virtualenv --python=python venv
   - source venv/bin/activate
   - python -V