Make atom.update() work.

src/app/atom.coffee

@@ -244,7 +244,7 @@ window.atom =
       windowState
 
   update: ->
-    throw new Error('atom.update is not implemented')
+    ipc.sendChannel 'install-update'
 
   getUpdateStatus: (callback) ->
     throw new Error('atom.getUpdateStatus is not implemented')

src/atom-application.coffee

@@ -35,6 +35,7 @@ class AtomApplication
   configWindow: null
   menu: null
   resourcePath: null
+  installUpdate: null
   version: null
 
   constructor: ({@resourcePath, pathsToOpen, @version, test, pidToKillWhenClosed, @dev}) ->
@@ -187,6 +188,7 @@ class AtomApplication
 
     autoUpdater.on 'ready-for-update-on-quit', (event, version, quitAndUpdate) =>
       event.preventDefault()
+      @installUpdate = quitAndUpdate
       @buildApplicationMenu version, quitAndUpdate
 
     ipc.on 'close-without-confirm', (processId, routingId) ->
@@ -206,6 +208,9 @@ class AtomApplication
     ipc.on 'new-window', =>
       @open()
 
+    ipc.on 'install-update', =>
+      @installUpdate?()
+
     ipc.on 'get-version', (event) =>
       event.result = @version