Prevent zombie processes from increasing

fixes #165

Before, in dashboard, clicking "Start" button creates a process as
zombie, but clicking "Stop" button doesn't kill it. This change prevents
it from becoming zombie.

app/models/fluentd/agent/fluentd_gem.rb

@@ -67,7 +67,8 @@ class Fluentd
       def actual_start
         return unless validate_fluentd_options
         Bundler.with_clean_env do
-          spawn("fluentd #{options_to_argv}")
+          pid = spawn("fluentd #{options_to_argv}")
+          Process.detach(pid)
         end
 
         wait_starting