Update babel to 7.8.7 graylog-web-plugin to fix a dependency issue

This fixes the following error when running "npm install" in
graylog-web-plugin:

  error: [BABEL] graylog2-web-interface/packages/graylog-web-plugin/src/PluginManifest.js:
    Could not find plugin "proposal-numeric-separator". Ensure there is an
    entry in ./available-plugins.js for it.

Related PR in babel: babel/babel#11201

Fixes #7731

graylog2-web-interface/packages/graylog-web-plugin/package.json

@@ -34,7 +34,7 @@
     "README.md"
   ],
   "dependencies": {
-    "@babel/preset-env": "7.8.4",
+    "@babel/preset-env": "7.8.7",
     "babel-eslint": "9.0.0",
     "eslint-config-graylog": "file:../eslint-config-graylog",
     "html-webpack-plugin": "3.2.0",
@@ -59,7 +59,7 @@
   },
   "devDependencies": {
     "@babel/cli": "^7.8.0",
-    "@babel/core": "^7.8.0",
+    "@babel/core": "^7.8.7",
     "babel-plugin-add-module-exports": "^1.0.2"
   }
 }