build(babel): now min node version is 6

Author: nodkz

.babelrc

@@ -7,7 +7,7 @@
   "presets": [
     ["env", {
       "targets": {
-        "node": 4
+        "node": 6
       },
     }]
   ],

.travis.yml

@@ -10,7 +10,6 @@ node_js:
   - "8"
   - "7"
   - "6"
-  - "4"
 before_install: yarn global add greenkeeper-lockfile@1
 before_script: greenkeeper-lockfile-update
 after_script: greenkeeper-lockfile-upload