Bukkit

Can you help me with this? Do I have to pay for this plugin? Why isn't it opening the .jar file?

[18:37:50 ERROR]: Could not load 'plugins/SurvivalGames.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: me/maker56/survivalgames/SurvivalGames : Unsupported major.minor version 51.0
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:137) ~[craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:313) ~[craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:236) [craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.craftbukkit.v1_7_R1.CraftServer.loadPlugins(CraftServer.java:350) [craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.craftbukkit.v1_7_R1.CraftServer.<init>(CraftServer.java:312) [craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at net.minecraft.server.v1_7_R1.PlayerList.<init>(PlayerList.java:63) [craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at net.minecraft.server.v1_7_R1.DedicatedPlayerList.<init>(SourceFile:14) [craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at net.minecraft.server.v1_7_R1.DedicatedServer.init(DedicatedServer.java:126) [craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at net.minecraft.server.v1_7_R1.MinecraftServer.run(MinecraftServer.java:424) [craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at net.minecraft.server.v1_7_R1.ThreadServerApplication.run(SourceFile:617) [craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
Caused by: java.lang.UnsupportedClassVersionError: me/maker56/survivalgames/SurvivalGames : Unsupported major.minor version 51.0
    at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.6.0_65]
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:637) ~[?:1.6.0_65]
    at java.lang.ClassLoader.defineClass(ClassLoader.java:621) ~[?:1.6.0_65]
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141) ~[?:1.6.0_65]
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:283) ~[?:1.6.0_65]
    at java.net.URLClassLoader.access$000(URLClassLoader.java:58) ~[?:1.6.0_65]
    at java.net.URLClassLoader$1.run(URLClassLoader.java:197) ~[?:1.6.0_65]
    at java.security.AccessController.doPrivileged(Native Method) ~[?:1.6.0_65]
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190) ~[?:1.6.0_65]
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:77) ~[craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:62) ~[craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306) ~[?:1.6.0_65]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247) ~[?:1.6.0_65]
    at java.lang.Class.forName0(Native Method) ~[?:1.6.0_65]
    at java.lang.Class.forName(Class.java:249) ~[?:1.6.0_65]
    at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:40) ~[craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:133) ~[craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]

The plugion only run in java 7. Do you use an older version?