Fix gradle build

Fixed `gradle build`. It does not include the lwjgl into the jar, though..

Running the jar given in `build/libs` gives this error:
```
I'm a little teapot. That is all.
Exception in thread "main" java.lang.UnsatisfiedLinkError: no lwjgl in java.library.path
	at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1758)
	at java.lang.Runtime.loadLibrary0(Runtime.java:823)
	at java.lang.System.loadLibrary(System.java:1045)
	at org.lwjgl.Sys$1.run(Sys.java:73)
	at java.security.AccessController.doPrivileged(Native Method)
	at org.lwjgl.Sys.doLoadLibrary(Sys.java:66)
	at org.lwjgl.Sys.loadLibrary(Sys.java:95)
	at org.lwjgl.Sys.<clinit>(Sys.java:112)
	at org.lwjgl.opengl.Display.<clinit>(Display.java:132)
	at org.newdawn.slick.AppGameContainer$1.run(AppGameContainer.java:37)
	at java.security.AccessController.doPrivileged(Native Method)
	at org.newdawn.slick.AppGameContainer.<clinit>(AppGameContainer.java:34)
	at com.mojang.mojam.MainClass.main(MainClass.java:10)
```