ci(Travis): drop node vesions 4,5

Author: nodkz

.travis.yml

@@ -9,8 +9,7 @@ notifications:
 node_js:
   - "8"
   - "7"
-  - "5"
-  - "4"
+  - "6"
 before_install: yarn global add greenkeeper-lockfile@1
 before_script: greenkeeper-lockfile-update
 after_script: greenkeeper-lockfile-upload