build(Babel): set node 8 as min build version

BREAKING CHANGE: Drop node 6 support.

Author: nodkz

.babelrc

@@ -13,7 +13,7 @@
           "@babel/preset-env",
           {
             "targets": {
-              "node": 6
+              "node": 8
             }
           }
         ]
@@ -28,7 +28,7 @@
           "@babel/preset-env",
           {
             "targets": {
-              "node": "8.0.0"
+              "node": 8
             },
             "loose": true,
             "modules": false
@@ -50,7 +50,7 @@
           "@babel/preset-env",
           {
             "targets": {
-              "node": "8.0.0"
+              "node": 8
             },
             "loose": true,
             "modules": "commonjs"