Drop Node.js < 8 support.

Node.js 6 goes EOL on April 2019.

.travis.yml

@@ -4,22 +4,22 @@ language: node_js
 git:
   depth: 3
 node_js:
-  - "6"
   - "8"
+  - "10"
 install:
   - bundle install --deployment --jobs=3 --retry=3 --clean
-  - npm install
+  - npm ci
 before_script:
   - google-chrome-stable --product-version
 script:
   - npm test || travis_terminate 1
-  - if [[ "$TRAVIS_NODE_VERSION" = "8" ]]; then npm run linkinator; fi
-  - if [[ "$TRAVIS_NODE_VERSION" = "8" && "$TRAVIS_EVENT_TYPE" = "push" && ! `git log --format=%B --no-merges -n 1 | grep '\[skip browser\]'` ]]; then npm run js-test-cloud; fi
+  - if [[ "$TRAVIS_NODE_VERSION" = "10" ]]; then npm run linkinator; fi
+  - if [[ "$TRAVIS_NODE_VERSION" = "10" && "$TRAVIS_EVENT_TYPE" = "push" && ! `git log --format=%B --no-merges -n 1 | grep '\[skip browser\]'` ]]; then npm run js-test-cloud; fi
 after_success:
-  - if [[ "$TRAVIS_NODE_VERSION" = "8" ]]; then npm run coveralls; fi
+  - if [[ "$TRAVIS_NODE_VERSION" = "10" ]]; then npm run coveralls; fi
 cache:
   directories:
-    - node_modules
+    - "$HOME/.npm"
     - vendor/bundle
 notifications:
   email: false

package.json

@@ -145,7 +145,7 @@
     "vnu-jar": "18.11.5"
   },
   "engines": {
-    "node": ">=6"
+    "node": ">=8"
   },
   "files": [
     "dist/{css,js}/*.{css,js,map}",