Bump backstopjs from 4.1.12 to 4.3.4

Bumps [backstopjs](https://github.com/garris/backstopjs) from 4.1.12 to 4.3.4.
- [Release notes](https://github.com/garris/backstopjs/releases)
- [Commits](garris/BackstopJS@v4.1.12...v4.3.4)

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

package.json

@@ -71,7 +71,7 @@
     "@typescript-eslint/eslint-plugin": "^2.0.0",
     "@typescript-eslint/parser": "^2.0.0",
     "babel-loader": "^8.0.6",
-    "backstopjs": "^4.1.12",
+    "backstopjs": "^4.3.4",
     "coveralls": "^3.0.6",
     "electron": "^6.0.4",
     "electron-builder": "^21.2.0",