Don't makeSingleInstance in a MAS build

It seems that makeSingleInstance will always return true in the MAS
build.* Luckily, OSX mostly enforces single-instance for us when opening
the app from Finder. If the user attempts to run a second instance from
the command line, it will create a window but fail to load.

*TODO: Revisit why makeSingleInstance is broken in the MAS build.

// FREEBIE

main.js

@@ -15,7 +15,8 @@ app.setAppUserModelId('org.whispersystems.signal-desktop')
 const package_json = JSON.parse(fs.readFileSync(path.join(__dirname, 'package.json'), 'utf-8'))
 const environment = package_json.environment || process.env.NODE_ENV || 'development';
 
-if (environment === 'production') {
+if (environment === 'production' && !process.mas) {
+  console.log('making app single instance');
   var shouldQuit = app.makeSingleInstance(function(commandLine, workingDirectory) {
     // Someone tried to run a second instance, we should focus our window
     if (mainWindow) {