Bukkit

What steps will reproduce the problem?
1.error in Log
2.
3.

What is the expected output? What do you see instead?

[09:58:22] [Server thread/ERROR]: Could not pass event PlayerJoinEvent to SetRankPEX v1.7.1
org.bukkit.event.EventException
    at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:305) ~[minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at net.minecraft.server.v1_8_R2.PlayerList.onPlayerJoin(PlayerList.java:296) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at net.minecraft.server.v1_8_R2.PlayerList.a(PlayerList.java:156) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at net.minecraft.server.v1_8_R2.LoginListener.b(LoginListener.java:144) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at net.minecraft.server.v1_8_R2.LoginListener.c(LoginListener.java:54) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at net.minecraft.server.v1_8_R2.NetworkManager.a(NetworkManager.java:231) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at net.minecraft.server.v1_8_R2.ServerConnection.c(ServerConnection.java:148) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at net.minecraft.server.v1_8_R2.MinecraftServer.A(MinecraftServer.java:809) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at net.minecraft.server.v1_8_R2.DedicatedServer.A(DedicatedServer.java:368) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at net.minecraft.server.v1_8_R2.MinecraftServer.z(MinecraftServer.java:651) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at net.minecraft.server.v1_8_R2.MinecraftServer.run(MinecraftServer.java:554) [minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at java.lang.Thread.run(Thread.java:745) [?:1.8.0_20]
Caused by: java.lang.NoClassDefFoundError: org/bukkit/craftbukkit/libs/com/google/gson/JsonParser
    at me.Irock23.SRPEX.SetRankPEX.checkVersion(SetRankPEX.java:150) ~[?:?]
    at me.Irock23.SRPEX.PlayerListener.onPlayerJoin(PlayerListener.java:23) ~[?:?]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_20]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_20]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_20]
    at java.lang.reflect.Method.invoke(Method.java:483) ~[?:1.8.0_20]
    at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:301) ~[minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    ... 14 more
Caused by: java.lang.ClassNotFoundException: org.bukkit.craftbukkit.libs.com.google.gson.JsonParser
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:91) ~[minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:86) ~[minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_20]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_20]
    at me.Irock23.SRPEX.SetRankPEX.checkVersion(SetRankPEX.java:150) ~[?:?]
    at me.Irock23.SRPEX.PlayerListener.onPlayerJoin(PlayerListener.java:23) ~[?:?]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_20]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_20]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_20]
    at java.lang.reflect.Method.invoke(Method.java:483) ~[?:1.8.0_20]
    at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:301) ~[minecraft_server.jar:git-Spigot-0b997a9-30d751b]
    ... 14 more