upgrade to electron@^6.1.7 with MAS patch

This version of Electron includes a patch which:
- removes the use of some private APIs which were causing Electron apps
to be rejected by the Mac App Store, but
- might cause degraded performance on Mac.

Author: Christopher Willis-Ford

package-lock.json

@@ -35,7 +35,7 @@
     "babel-loader": "^8.0.6",
     "babel-plugin-react-intl": "^2.3.1",
     "copy-webpack-plugin": "^4.6.0",
-    "electron": "^4.2.9",
+    "electron": "^6.1.7",
     "electron-builder": "^20.44.4",
     "electron-devtools-installer": "^2.2.4",
     "electron-store": "^3.3.0",