Bump node dependency to 8.9.0

6.9.0 is EOL next month and our defaultConfig stub already uses features not supported in 6.x, which means I either had to change that file to use Object.assign instead of spread, or bump our dependency.

Would rather not have to write prehistoric JS just to support an almost-EOL version.

package.json

@@ -63,7 +63,7 @@
         "@babel/preset-env",
         {
           "targets": {
-            "node": "6.9.0"
+            "node": "8.9.0"
           }
         }
       ]
@@ -78,6 +78,6 @@
     ]
   },
   "engines": {
-    "node": ">=6.9.0"
+    "node": ">=8.9.0"
   }
 }

stubs/defaultConfig.stub.js

@@ -236,9 +236,10 @@ module.exports = {
       '4': '4px',
       '8': '8px',
     },
-    borderColor: theme => {
-      return global.Object.assign({ default: theme('colors.gray.300', 'currentColor') }, theme('colors'))
-    },
+    borderColor: theme => ({
+      ...theme('colors'),
+      default: theme('colors.gray.300', 'currentColor'),
+    }),
     borderRadius: {
       none: '0',
       sm: '.125rem',