Bump flatpickr from 4.5.2 to 4.5.7

Bumps [flatpickr](https://github.com/chmln/flatpickr) from 4.5.2 to 4.5.7.
- [Release notes](https://github.com/chmln/flatpickr/releases)
- [Commits](https://github.com/chmln/flatpickr/compare/v4.5.2...v4.5.7)

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

package.json

@@ -17,7 +17,7 @@
     "chart.js": "^2.7.0",
     "cross-env": "^5.1",
     "es5-shim": "^4.5.9",
-    "flatpickr": "^4.5.0",
+    "flatpickr": "^4.5.7",
     "github-markdown-css": "^2.3.0",
     "ionicons": "~2.0",
     "jquery": "^2.2.4",