Also fix run_config not being resolved and make it a more proper fix

lib/requirejs/rails/config.rb

@@ -100,21 +100,23 @@ def loader=(sym)
       end
 
       def build_config
-        self[:build_config] = self.run_config.merge "baseUrl" => source_dir.to_s,
-                                                    "modules" => [{'name' => 'application'}]
-        self[:build_config].merge!(self.user_config).slice!(*self.build_config_whitelist)
-        case self.loader
-          when :requirejs
-            # nothing to do
-          when :almond
-            mods = self[:build_config]['modules']
-            unless mods.length == 1
-              raise Requirejs::ConfigError, "Almond build requires exactly one module, config has #{mods.length}."
-            end
-            mod = mods[0]
-            name = mod['name']
-            mod['name'] = 'almond'
-            mod['include'] = name
+        unless self.has_key?(:build_config)
+          self[:build_config] = self.run_config.merge "baseUrl" => source_dir.to_s,
+                                                      "modules" => [{'name' => 'application'}]
+          self[:build_config].merge!(self.user_config).slice!(*self.build_config_whitelist)
+          case self.loader
+            when :requirejs
+              # nothing to do
+            when :almond
+              mods = self[:build_config]['modules']
+              unless mods.length == 1
+                raise Requirejs::ConfigError, "Almond build requires exactly one module, config has #{mods.length}."
+              end
+              mod = mods[0]
+              name = mod['name']
+              mod['name'] = 'almond'
+              mod['include'] = name
+          end
         end
         self[:build_config]
       end
@@ -131,6 +133,9 @@ def user_config=(cfg)
         if url = cfg.delete('baseUrl')
           raise Requirejs::ConfigError, "baseUrl is not needed or permitted in the configuration"
         end
+        # delete any cached configs, will be lazily resolved
+        self.delete(:build_config)
+        self.delete(:run_config)
         self[:user_config] = cfg
       end