A server is already running. Check tmp/pids/server.pid.
把server.pid删除；
学习了：
http://stackoverflow.com/questions/24627701/a-server-is-already-running-check-tmp-pids-server-pid-exiting-rails
A server is already running. Check tmp/pids/server.pid.
原文：http://www.cnblogs.com/stono/p/6819419.html