ci(.travis.yml): stop testing on anything but node 7

Author: jedwards1211

.travis.yml

@@ -4,10 +4,7 @@ cache:
     - node_modules
 notifications:
   email: false
-node_js:
-  - '7'
-  - '6'
-  - '4'
+node_js: 7
 before_script:
   - npm prune
 script:
@@ -17,3 +14,4 @@ after_success:
 branches:
   except:
     - /^v\d+\.\d+\.\d+$/
+