chore: run travis on newer Node versions, cache node_modules

Author: ZauberNerd

.travis.yml

@@ -1,8 +1,11 @@
-sudo: required
 language: node_js
 node_js:
   - 4
-  - 5
-  - stable
-script:
-  - npm run test
+  - 6
+  - 8
+  - 10
+cache:
+  directories:
+    - ".eslintcache"
+    - "node_modules"
+script: npm test