.travis.yml: Add sentinel job

.travis.yml

@@ -6,27 +6,49 @@ python:
   - 3.5
   - 3.6
 
+.disable_global: &disable_global
+  addons: false
+  cache: pip
+  before_install: false
+  install: false
+  before_script: false
+  script: false
+  after_success: false
+  after_failure: false
+  before_deploy: false
+  deploy: false
+
 stages:
+  - name: sentinel
+    if: branch != master OR type = pull_request
   - test
   - moban
   - name: unsupported
     if: branch = master AND type = push
 
 .check_moban: &check_moban
+  <<: *disable_global
   python: 3.6
   stage: moban
-  addons: false
-  cache: pip
-  before_install: false
   install: pip install moban
-  before_script: false
   script: .ci/check_moban.sh
-  after_success: false
-  after_failure: false
   if: branch != master OR type = pull_request
 
 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