run exception on pf4j-spring demo

[3fong]

when very first run pf4j-spring demo you will get some fails:
1 if you use some high jdk version ,like 11.0.13,then you will get the fail: package javax.annotation does not exist in .
error detail:

[ERROR] COMPILATION ERROR :
[INFO] -------------------------------------------------------------
[ERROR] /D:/git_repo/demo/pf4j-spring/pf4j-spring/src/main/java/org/pf4j/spring/SpringPluginManager.java:[25,24] 程序包javax.annotation不存在
[ERROR] /D:/git_repo/demo/pf4j-spring/pf4j-spring/src/main/java/org/pf4j/spring/SpringPluginManager.java:[65,6] 找不到符号
  符号:   类 PostConstruct
  位置: 类 org.pf4j.spring.SpringPluginManager

2 lack the reference of java.util.List in org.pf4j.spring.SpringPluginManager