Bump popper.js from 1.14.7 to 1.15.0

Bumps [popper.js](https://github.com/FezVrasta/popper.js) from 1.14.7 to 1.15.0.
- [Release notes](https://github.com/FezVrasta/popper.js/releases)
- [Changelog](https://github.com/FezVrasta/popper.js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/FezVrasta/popper.js/compare/v1.14.7...v1.15.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -12,7 +12,7 @@
     "jquery-ui": "^1.12.1",
     "jstree": "^3.3.5",
     "opensans-webkit": "^1.0.1",
-    "popper.js": "^1.14.4",
+    "popper.js": "^1.15.0",
     "underscore": "^1.9.1",
     "vis": "^4.21.0",
     "webpack-merge": "^4.1.4"