Bump @babel/core from 7.5.5 to 7.6.3 in /assets

Bumps [@babel/core](https://github.com/babel/babel) from 7.5.5 to 7.6.3.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](babel/babel@v7.5.5...v7.6.3)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

assets/package-lock.json

@@ -10,7 +10,7 @@
     "phoenix_html": "file:../deps/phoenix_html"
   },
   "devDependencies": {
-    "@babel/core": "^7.5.5",
+    "@babel/core": "^7.6.3",
     "@babel/preset-env": "^7.6.3",
     "babel-loader": "^8.0.6",
     "copy-webpack-plugin": "^5.0.4",