Add enzyme to dev dependencies

Author: ryota-ka

package.json

@@ -29,6 +29,7 @@
     "@types/mocha": "^5.2.7",
     "@types/node": "^10.5.2",
     "@types/react": "16.9.3",
+    "enzyme": "^3.11.0",
     "mocha": "^6.2.0",
     "react": "16.9.0",
     "react-dom": "16.9.0",