Change Travis file to test against Python nightly

Change Travis file to test against latest nightly rather than a fixed
Python 3 version so that it doesn't have to be manually changed.

Author: tofu-rocketry

.travis.yml

@@ -2,10 +2,10 @@ language: python
 python:
   - "2.6"
   - "2.7"
-  - "3.5"
+  - "nightly"
 matrix:
   allow_failures:
-    - python: "3.5"
+    - python: "nightly"
   fast_finish: true
 
 # Route build to container-based infrastructure