.travis.yml: Add sentinel job for coala-ci

This adds a Python 3.6 'coala-ci' sentinel job for builds
except pushes to master, so it runs by itself initially
to check code style only, and causes all other jobs to
be cancelled if it fails.

.travis.yml

@@ -19,6 +19,8 @@ python:
   deploy: false
 
 stages:
+  - name: sentinel
+    if: branch != master OR type = pull_request
   - test
   - moban
   - name: unsupported
@@ -34,6 +36,19 @@ stages:
 
 jobs:
   include:
+    - stage: sentinel
+      # All other jobs will be cancelled if the sentinel job fails
+      <<: *disable_global
+      python: 3.6
+      install: pip install ".[alldeps]"
+      script: coala --non-interactive -V
+
+    - stage: test
+      # The sentinel job isnt active on master pushes, so the
+      # job is re-added to the bottom of the test matrix
+      python: 3.6
+      if: branch = master AND type = push
+
     - python: 2.7
       stage: unsupported
       env: PIP_NO_COMPILE=1