Revert electron ~7.0.0-beta.1 to ^6.0.0

The beta was previously needed for ELECTRON_SKIP_BINARY_DOWNLOAD,
see 975ea2a. Electron folks kindly backported that feature to v6.

Ref #678
Ref electron/electron#20438

package.json

@@ -37,7 +37,7 @@
     "delayed": "^2.0.0",
     "dependency-check": "^4.1.0",
     "du": "^1.0.0",
-    "electron": "~7.0.0-beta.1",
+    "electron": "^6.0.12",
     "faucet": "^0.0.1",
     "glob": "^7.1.3",
     "hallmark": "^2.0.0",