Remove log directory in test postprocessing

The "log" is directory. So it is necessary to use `FileUtils.rm_rf`.

actionview/Rakefile

@@ -47,7 +47,7 @@ namespace :test do
       status = $?.to_i
     ensure
       Process.kill("KILL", pid) if pid
-      FileUtils.rm_f("log")
+      FileUtils.rm_rf("log")
     end
 
     exit status