Add conditional to download and apply RELEASES file on Windows

if he braveURL attribute is set in the metadata respone. This will
download the version specified, not the latest version is happening
before this commit.

Auditors: @bbondy, @clifton

Test plan:

  * Change version in package.json to something lower than current
  * Build browser on Windows
  * Install and check for updates
  * Ensure the following line is shown in SquirrelSetup.log
    It must contain a version number (0.19.16 in example below)

2017-09-25 18:04:13> CheckForUpdateImpl: Downloading RELEASES file from
https://brave-download.global.ssl.fastly.net/multi-channel/releases/beta/0.19.16/winx64

app/updater.js

@@ -187,6 +187,9 @@ var downloadHandler = (err, metadata) => {
   if (process.platform === 'win32') {
     // check versions to see if an update is required
     if (metadata) {
+      if (metadata.braveURL) {
+        autoUpdater.setFeedURL(metadata.braveURL)
+      }
       autoUpdater.checkForUpdates()
     } else {
       autoUpdater.emit(messages.UPDATE_NOT_AVAILABLE)