chore(travis): run in Node 8, cache ~/.npm

Author: gr2m

.travis.yml

@@ -2,11 +2,11 @@ sudo: false
 language: node_js
 cache:
   directories:
-    - node_modules
+    - ~/.npm
 notifications:
   email: false
 node_js:
-  - 4
+  - 8
 script: npm run $COMMAND
 before_script:
   - npm prune