itemstackererror

20:49:42 [SEVERE] Could not load 'plugins/ItemStacker.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: me/ryvix/ItemStacker/ItemStacker : Unsupported major.minor version 51.0
	at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:155)
	at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:305)
	at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:230)
	at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:222)
	at org.bukkit.craftbukkit.CraftServer.reload(CraftServer.java:565)
	at org.bukkit.Bukkit.reload(Bukkit.java:183)
	at org.bukkit.command.defaults.ReloadCommand.execute(ReloadCommand.java:21)
	at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:168)
	at org.bukkit.craftbukkit.CraftServer.dispatchCommand(CraftServer.java:492)
	at org.bukkit.craftbukkit.CraftServer.dispatchServerCommand(CraftServer.java:488)
	at net.minecraft.server.DedicatedServer.ai(DedicatedServer.java:249)
	at net.minecraft.server.DedicatedServer.q(DedicatedServer.java:214)
	at net.minecraft.server.MinecraftServer.p(MinecraftServer.java:474)
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:406)
	at net.minecraft.server.ThreadServerApplication.run(SourceFile:539)
Caused by: java.lang.UnsupportedClassVersionError: me/ryvix/ItemStacker/ItemStacker : Unsupported major.minor version 51.0
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
	at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
	at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:44)
	at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:29)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:264)
	at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:144)
	... 14 more