use build stages for Travis CI (mochajs#3302)

see https://docs.travis-ci.com/user/build-stages/

.travis.yml

@@ -6,25 +6,37 @@ language: node_js
 
 matrix:
   fast_finish: true
+
+before_install: scripts/travis-before-install.sh
+
+before_script: scripts/travis-before-script.sh
+
+jobs:
   include:
-    - node_js: '9'
+    - stage: lint
+      node_js: '8'
+      env: TARGET=lint
+      script: npm start $TARGET
+    - stage: test
+      node_js: '9'
       env: TARGET=test.node COVERAGE=true
+      script: npm start $TARGET
     - node_js: '8'
       env: TARGET=test.node
+      script: npm start $TARGET
     - node_js: '6'
       env: TARGET=test.node
+      script: npm start $TARGET
     - node_js: '4'
       env: TARGET=test.node
-    - node_js: '8'
-      env: TARGET=lint
+      script: npm start $TARGET
     - node_js: '8'
       env: TARGET=test.browser
+      script: npm start $TARGET
 
-before_install: scripts/travis-before-install.sh
-
-before_script: scripts/travis-before-script.sh
-
-script: npm start $TARGET
+stages:
+  - lint
+  - test
 
 after_success: npm start coveralls