1.8 incopatibility with double hands

[SnowyCoder]

In GuiEventListener.java, onPlayerInteract uses some APIs only made for spigot >= 1.9 for checking that the event doesn't get fired two times (eventually opening two GUIs).
If this usage could be removed or restricted for his own versions the backward-spigot compatibiliy could be lowered more

[21:38:04 ERROR]: Could not pass event PlayerInteractEvent to GuiApi v1.0-SNAPSHOT org.bukkit.event.EventException at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:305) ~[spigot.jar:git-Spigot-550ebace-7019900e] at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot.jar:git-Spigot-550ebace-7019900e] at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot.jar:git-Spigot-550ebace-7019900e] at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot.jar:git-Spigot-550ebace-7019900e] at org.bukkit.craftbukkit.v1_8_R1.event.CraftEventFactory.callPlayerInteractEvent(CraftEventFactory.java:226) [spigot.jar:git-Spigot-550ebace-7019900e] at org.bukkit.craftbukkit.v1_8_R1.event.CraftEventFactory.callPlayerInteractEvent(CraftEventFactory.java:193) [spigot.jar:git-Spigot-550ebace-7019900e] at org.bukkit.craftbukkit.v1_8_R1.event.CraftEventFactory.callPlayerInteractEvent(CraftEventFactory.java:189) [spigot.jar:git-Spigot-550ebace-7019900e] at net.minecraft.server.v1_8_R1.PlayerConnection.a(PlayerConnection.java:682) [spigot.jar:git-Spigot-550ebace-7019900e] at net.minecraft.server.v1_8_R1.PacketPlayInBlockPlace.a(PacketPlayInBlockPlace.java:50) [spigot.jar:git-Spigot-550ebace-7019900e] at net.minecraft.server.v1_8_R1.PacketPlayInBlockPlace.a(PacketPlayInBlockPlace.java:80) [spigot.jar:git-Spigot-550ebace-7019900e] at net.minecraft.server.v1_8_R1.PacketHandleTask.run(SourceFile:13) [spigot.jar:git-Spigot-550ebace-7019900e] at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_121] at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_121] at net.minecraft.server.v1_8_R1.MinecraftServer.z(MinecraftServer.java:696) [spigot.jar:git-Spigot-550ebace-7019900e] at net.minecraft.server.v1_8_R1.DedicatedServer.z(DedicatedServer.java:316) [spigot.jar:git-Spigot-550ebace-7019900e] at net.minecraft.server.v1_8_R1.MinecraftServer.y(MinecraftServer.java:634) [spigot.jar:git-Spigot-550ebace-7019900e] at net.minecraft.server.v1_8_R1.MinecraftServer.run(MinecraftServer.java:537) [spigot.jar:git-Spigot-550ebace-7019900e] at java.lang.Thread.run(Thread.java:745) [?:1.8.0_121] Caused by: java.lang.NoSuchMethodError: org.bukkit.event.player.PlayerInteractEvent.getHand()Lorg/bukkit/inventory/EquipmentSlot; at xyz.upperlevel.spigot.gui.GuiEventListener.onPlayerInteract(GuiEventListener.java:73) ~[?:?] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_121] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_121] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_121] at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_121] at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:301) ~[spigot.jar:git-Spigot-550ebace-7019900e] ... 17 more