Make sure that the watcher gets started

If loading spring/commands (which loads the gemfile / spring.rb / etc)
fails, we still need a watcher. It might end up being the polling watcher
when the user has configured the listen watcher, but that's acceptable.

Author: jonleighton

lib/spring/application.rb

@@ -68,8 +68,11 @@ def start_watcher
     def preload
       log "preloading app"
 
-      require "spring/commands"
-      start_watcher
+      begin
+        require "spring/commands"
+      ensure
+        start_watcher
+      end
 
       require Spring.application_root_path.join("config", "application")