Having this error trying to load the plugin:

Code: 08:00:05 [SEVERE] Could not load 'plugins/BookDupe.jar' in folder 'plugins' org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: com/github/Indiv0/BookDupe/BookDupe : 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.<init>(CraftServer.java:198) at net.minecraft.server.ServerConfigurationManagerAbstract.<init>(ServerConfigurationManagerAbstract.java:50) at net.minecraft.server.ServerConfigurationManager.<init>(SourceFile:11) at net.minecraft.server.DedicatedServer.init(DedicatedServer.java:105) at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:380) at net.minecraft.server.ThreadServerApplication.run(SourceFile:539) Caused by: java.lang.UnsupportedClassVersionError: com/github/Indiv0/BookDupe/BookDupe : Unsupported major.minor version 51.0 at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631) at java.lang.ClassLoader.defineClass(ClassLoader.java:615) at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141) at java.net.URLClassLoader.defineClass(URLClassLoader.java:283) at java.net.URLClassLoader.access$000(URLClassLoader.java:58) at java.net.URLClassLoader$1.run(URLClassLoader.java:197) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:41) at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:29) at java.lang.ClassLoader.loadClass(ClassLoader.java:306) at java.lang.ClassLoader.loadClass(ClassLoader.java:247) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Class.java:247) at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:144) ... 9 more