Bug 1376586 - Add back quitApplication command alias; r=davehunt

The quitApplication command was accidentally removed in 1e96a289d28a.
This adds it back as a recognised command alias in order to not break
geckodriver.

MozReview-Commit-ID: IZ0h8dv9ILt

--HG--
extra : rebase_source : 61550dae3a71973a2028d6384bc3eab388c17c21

testing/marionette/driver.js

@@ -3265,6 +3265,7 @@ GeckoDriver.prototype.commands = {
   "acceptConnections": GeckoDriver.prototype.acceptConnections,  // deprecated, remove in Firefox 60
   "Marionette:Quit": GeckoDriver.prototype.quit,
   "quit": GeckoDriver.prototype.quit,  // deprecated, remove in Firefox 60
+  "quitApplication": GeckoDriver.prototype.quit,  // deprecated, remove in Firefox 60
 
   // Addon service
   "Addon:Install": GeckoDriver.prototype.installAddon,