Bukkit

What steps will reproduce the problem?
1. Place plugin in plugins folder
2. Start Spigot server, for Minecraft 1.4.7
3. Plugin doesn't load. "Unsupported major.minor version 51.0"

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

See error log below.

What version of the product are you using?
v1.2

Do you have an error log of what happened?

Here is the output from the server console:

23:30:45 [SEVERE] Could not load 'plugins/LevelUpRewards.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: com/outlook/mydayyy/LevelUpRewards : Unsupported major.minor version 51.0
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:184)
    at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:307)
    at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:231)
    at org.bukkit.craftbukkit.v1_4_R1.CraftServer.loadPlugins(CraftServer.java:261)
    at org.bukkit.craftbukkit.v1_4_R1.CraftServer.<init>(CraftServer.java:239)
    at net.minecraft.server.v1_4_R1.PlayerList.<init>(PlayerList.java:55)
    at net.minecraft.server.v1_4_R1.DedicatedPlayerList.<init>(SourceFile:11)
    at net.minecraft.server.v1_4_R1.DedicatedServer.init(DedicatedServer.java:108)
    at net.minecraft.server.v1_4_R1.MinecraftServer.run(MinecraftServer.java:405)
    at net.minecraft.server.v1_4_R1.ThreadServerApplication.run(SourceFile:849)
Caused by: java.lang.UnsupportedClassVersionError: com/outlook/mydayyy/LevelUpRewards : 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.findClass0(PluginClassLoader.java:87)
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:53)
    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:249)
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:173)
    ... 9 more

Please provide any additional information below.

I am running a Spigot server, but I also tested the plugin in Bukkit for Minecraft 1.4.7, and got the same error.