Removed outdated Node version tests

Author: ZeeCoder

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 sudo: false
-node_js: [ 4, 5, 6, 7, 8 ]
+node_js: [ 6, 7, 8 ]
 after_success: 'yarn run coveralls'
 cache:
     yarn: true