Bump bootstrap from 4.1.1 to 4.3.1

Bumps [bootstrap](https://github.com/twbs/bootstrap) from 4.1.1 to 4.3.1.
- [Release notes](https://github.com/twbs/bootstrap/releases)
- [Commits](https://github.com/twbs/bootstrap/compare/v4.1.1...v4.3.1)

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

package.json

@@ -32,7 +32,7 @@
     "babel-core": "^6.26.3",
     "babel-loader": "^7.1.5",
     "body-parser": "^1.18.3",
-    "bootstrap": "4.1.1",
+    "bootstrap": "4.3.1",
     "bootswatch": "^4.1.1",
     "browser-sync": "^2.24.5",
     "css-loader": "^1.0.0",