Bump @babel/core from 7.5.5 to 7.6.0 in /app (#203)

Bumps [@babel/core](https://github.com/babel/babel) from 7.5.5 to 7.6.0.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](babel/babel@v7.5.5...v7.6.0)

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Author: dependabot-preview[bot]

app/package-lock.json

@@ -10,7 +10,7 @@
     "turbolinks": "^5.2.0"
   },
   "devDependencies": {
-    "@babel/core": "^7.5.5",
+    "@babel/core": "^7.6.0",
     "@babel/plugin-proposal-class-properties": "^7.5.5",
     "@babel/preset-env": "^7.5.5",
     "@fullhuman/postcss-purgecss": "^1.2.0",