Upgrade electron

Summary:
Tried to upgrade both electron and electron-builder but failed due to Windows binary build stuck.

Upgraded only electron for now. electron-builder will be upgraded separately

Reviewed By: nikoant

Differential Revision: D19877865

fbshipit-source-id: a2282c2f4062e6b470cdcba2e9e693419bfeaec4

scripts/build-utils.js

@@ -106,7 +106,7 @@ function getVersionNumber() {
 // Asynchronously determine current mercurial revision as string or `null` in case of any error.
 function genMercurialRevision() {
   return cp
-    .spawn('hg', ['log', '-r', '.', '-T', '{node}'])
+    .spawn('hg', ['log', '-r', '.', '-T', '{node}'], {encoding: 'utf8'})
     .catch(err => null)
     .then(res => (res && res.stdout) || null);
 }