Build: Only run browser tests in one Node version on Travis

Ref gh-3744
Closes gh-3894

.travis.yml

@@ -7,3 +7,12 @@ node_js:
 - "9"
 addons:
   chrome: stable
+env:
+  - NPM_SCRIPT=test:browserless
+matrix:
+  include:
+    # Run browser tests only on one Node.js version to save time.
+    - node_js: "8"
+      env: NPM_SCRIPT=test:browser
+script:
+  - npm run $NPM_SCRIPT

package.json

@@ -68,8 +68,10 @@
   "scripts": {
     "build": "npm install && grunt",
     "start": "grunt watch",
-    "test": "grunt && grunt test:slow karma:main",
-    "jenkins": "grunt && grunt test:slow",
+    "test:browserless": "grunt && grunt test:slow",
+    "test:browser": "grunt && grunt karma:main",
+    "test": "grunt && grunt test:slow && grunt karma:main",
+    "jenkins": "npm run test:browserless",
     "precommit": "grunt lint:newer qunit_fixture",
     "commitmsg": "node node_modules/commitplease"
   },