ci(travis): remove testing in Node 6

Author: gr2m

.travis.yml

@@ -1,13 +1,11 @@
 language: node_js
-cache:
-  directories:
-    - ~/.npm
+cache: npm
 notifications:
   email: false
 node_js:
   - '10'
   - '8'
-  - '6'
+
 after_success:
   - npm run semantic-release
 # Trigger a push build on master and greenkeeper branches + PRs build on every branches