ci(travis): only run release stage on master push

.travis.yml

@@ -2,13 +2,8 @@ language: node_js
 cache:
   directories:
     - ~/.npm
-notifications:
-  email: false
 
-node_js:
-  - '10'
-  - '8'
-  - '6'
+node_js: lts/*
 
 # Trigger a push build on master and greenkeeper branches + PRs build on every branches
 # Avoid double build on PRs (See https://github.com/travis-ci/travis-ci/issues/1147)
@@ -17,13 +12,20 @@ branches:
     - master
     - /^greenkeeper.*$/
 
+stages:
+  - test
+  - name: release
+    if: branch = master AND type IN (push)
+
 jobs:
   include:
+    - stage: test
+      node_js: 6
+    - node_js: 8
+    - node_js: 10
     # run semantic-release and coverage in paralell
     - stage: release
-      node_js: lts/*
       script: npm run semantic-release
-    - node_js: lts/*
-      script:
+    - script:
         - npm run test
         - npm run coverage:upload