chore(travis): speed up travis build

Author: KyleRoss

.travis.yml

@@ -4,20 +4,15 @@ language: node_js
 node_js:
   - "11"
   - "10"
-  - "9"
   - "8"
   - "6"
-  - "6.10"
 after_success: 'npm run coveralls'
 
 os:
   - linux
 
 jobs:
   include:
-    - stage: install
-      script: npm install
-      skip_cleanup: true
     - stage: test
       script: npm run test
       skip_cleanup: true