Allow failures for dev versions of python

Author: Byron

.travis.yml

@@ -10,9 +10,11 @@ python:
   - "3.7-dev"
   - "nightly"
   # - "pypy" - won't work as smmap doesn't work (see gitdb/.travis.yml for details)
-#matrix:
-#  allow_failures:
-#    - python: "2.6"
+matrix:
+  allow_failures:
+    - python: "3.6-dev"
+    - python: "3.7-dev"
+    - python: "nightly"
 git:
   # a higher depth is needed for most of the tests - must be high enough to not actually be shallow
   # as we clone our own repository in the process