.travis.yml: Add sentinel job

This makes Python 3.6 the sentinel job,
so it runs by itself initially and causes
all other jobs to be cancelled if it fails.

.travis.yml

@@ -4,9 +4,9 @@ language: python
 python:
   - 3.4
   - 3.5
-  - 3.6
 
 stages:
+  - sentinel
   - test
   - moban
   - name: unsupported
@@ -26,7 +26,12 @@ stages:
   if: branch != master OR type = pull_request
 
 jobs:
+  # Any pending jobs will be cancelled if the sentinel job fails
+  fast_finish: true
   include:
+    - stage: sentinel
+      python: 3.6
+
     - python: 2.7
       stage: unsupported
       env: PIP_NO_COMPILE=1