Add loading gif for installer

build/Gruntfile.coffee

@@ -203,6 +203,7 @@ module.exports = (grunt) ->
       appDirectory: shellAppDir
       outputDirectory: path.join(buildDir, 'installer')
       authors: 'The Atom Team & Community'
+      loadingGif: path.resolve(__dirname, '..', 'resources', 'win', 'loading.gif')
       iconUrl: 'https://raw.githubusercontent.com/atom/atom/master/resources/atom.png'
 
     shell: