Fix support for rails engines/plugins

See here for a description of the problem:

#144 (comment)

The solution is to make the application_id depend on the *project* root
rather than the *application* root, since only the server can know the
correct application root.

Author: jonleighton

CHANGELOG.md

@@ -23,6 +23,7 @@
   cause spring to simply quit, meaning that you'd incur the full startup
   penalty on the next run. Now spring doesn't quit, and will try to load
   up your new bundle in the background.
+* Fix support for using spring with Rails engines/plugins
 
 ## 1.0.0
 

lib/spring/configuration.rb

@@ -22,15 +22,24 @@ def verify_environment
 
     def application_root_path
       @application_root_path ||= begin
-        path = Pathname.new(File.expand_path(application_root || find_project_root))
+        if application_root
+          path = Pathname.new(File.expand_path(application_root))
+        else
+          path = project_root_path
+        end
+
         raise MissingApplication.new(path) unless path.join("config/application.rb").exist?
         path
       end
     end
 
+    def project_root_path
+      @project_root_path ||= find_project_root(Pathname.new(File.expand_path(Dir.pwd)))
+    end
+
     private
 
-    def find_project_root(current_dir = Pathname.new(Dir.pwd))
+    def find_project_root(current_dir)
       if current_dir.join(gemfile).exist?
         current_dir
       elsif current_dir.root?

lib/spring/env.rb

@@ -33,7 +33,7 @@ def tmp_path
     end
 
     def application_id
-      Digest::MD5.hexdigest(root.to_s)
+      Digest::MD5.hexdigest(Spring.project_root_path.to_s)
     end
 
     def socket_path