Chore: modify travis.yml

Author: mysticatea

.travis.yml

@@ -1,8 +1,21 @@
 sudo: false
+
 language: node_js
 node_js:
   - "4"
   - "6"
   - "8"
+
+env:
+  - ESLINT_VERSION=3
+  - ESLINT_VERSION=4
+
+cache:
+  directories:
+    - node_modules
+
+before_script:
+  - npm i --no-save eslint@$ESLINT_VERSION; true
+
 after_success:
   - npm run codecov