Fix relative root for .jar

Fixed this error when running rake jar

Exception in thread "LWJGL Application" com.badlogic.gdx.utils.GdxRuntimeExcepti
on: com.badlogic.gdx.utils.GdxRuntimeException: Couldn't load file: recf/res/ima
ges/bg.png
        at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplicati
on.java:111)
Caused by: com.badlogic.gdx.utils.GdxRuntimeException: Couldn't load file: recf/
res/images/bg.png
        at com.badlogic.gdx.graphics.Pixmap.<init>(Pixmap.java:140)
        at com.badlogic.gdx.graphics.glutils.FileTextureData.prepare(FileTexture
Data.java:64)
        at com.badlogic.gdx.graphics.Texture.load(Texture.java:175)
        at com.badlogic.gdx.graphics.Texture.create(Texture.java:159)
        at com.badlogic.gdx.graphics.Texture.<init>(Texture.java:133)
        at com.badlogic.gdx.graphics.Texture.<init>(Texture.java:122)