chore(package): update @babel/plugin-proposal-class-properties to version 7.7.0

Closes #137

Author: greenkeeper[bot]

package.json

@@ -42,7 +42,7 @@
   },
   "devDependencies": {
     "@babel/core": "^7.7.0",
-    "@babel/plugin-proposal-class-properties": "7.5.5",
+    "@babel/plugin-proposal-class-properties": "7.7.0",
     "@babel/preset-env": "^7.5.5",
     "@babel/preset-react": "7.0.0",
     "@storybook/addon-actions": "^5.1.9",