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

@@ -20,3 +20,7 b' after_success:'
     - cp /tmp/ipy_coverage.xml ./
     - cp /tmp/.coverage ./
     - codecov
+
+matrix:
+    allow_failures:
+        python: nightly