Bukkit

When I start up my Bukkit server running 1.7.2-R0.3, I get a InvaildPluginException.

Log:

[18:57:30] [Server thread/ERROR]: Could not load 'plugins/ColoredSigns.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: de/janomine/coloredsigns/main/ColoredSigns : Unsupported major.minor version 51.0
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:137) ~[Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:313) ~[Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:236) [Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.craftbukkit.v1_7_R1.CraftServer.loadPlugins(CraftServer.java:350) [Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.craftbukkit.v1_7_R1.CraftServer.<init>(CraftServer.java:312) [Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at net.minecraft.server.v1_7_R1.PlayerList.<init>(PlayerList.java:63) [Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at net.minecraft.server.v1_7_R1.DedicatedPlayerList.<init>(SourceFile:14) [Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at net.minecraft.server.v1_7_R1.DedicatedServer.init(DedicatedServer.java:126) [Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at net.minecraft.server.v1_7_R1.MinecraftServer.run(MinecraftServer.java:424) [Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at net.minecraft.server.v1_7_R1.ThreadServerApplication.run(SourceFile:617) [Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
Caused by: java.lang.UnsupportedClassVersionError: de/janomine/coloredsigns/main/ColoredSigns : Unsupported major.minor version 51.0
    at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.6.0_30]
    at java.lang.ClassLoader.defineClass(ClassLoader.java:643) ~[?:1.6.0_30]
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) ~[?:1.6.0_30]
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:277) ~[?:1.6.0_30]
    at java.net.URLClassLoader.access$000(URLClassLoader.java:73) ~[?:1.6.0_30]
    at java.net.URLClassLoader$1.run(URLClassLoader.java:212) ~[?:1.6.0_30]
    at java.security.AccessController.doPrivileged(Native Method) ~[?:1.6.0_30]
    at java.net.URLClassLoader.findClass(URLClassLoader.java:205) ~[?:1.6.0_30]
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:77) ~[Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:62) ~[Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:323) ~[?:1.6.0_30]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:268) ~[?:1.6.0_30]
    at java.lang.Class.forName0(Native Method) ~[?:1.6.0_30]
    at java.lang.Class.forName(Class.java:270) ~[?:1.6.0_30]
    at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:40) ~[Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:133) ~[Craftbukkit-1.7.2:git-Bukkit-1.7.2-R0.3-b3020jnks]
    ... 9 more