Removing node v4 and v6 for Travis build

.travis.yml

 language: node_js
 node_js:
 - '7'
-- '6'
-- '4'
 addons:
   apt:
     sources:

test/security.js

@@ -35,5 +35,5 @@ describe('Security', () => {
     return snyk('./').then(report => {
       expect(report).toPassSnyk()
     })
-  }, 20000)
+  }, 30000)
 })