Just to make @AstroSnail happy :P

data/shaders/skybox.frag

@@ -1,8 +1,7 @@
-#version 130
+#version 110
 
-out vec4 out_Color; // Get rid of out keyword then set version to 110
 varying vec3 cardial_color;
 
 void main(void) {
-	out_Color = vec4(cardial_color.xyz, 1.0f);
+	gl_FragColor = vec4(cardial_color.xyz, 1.0f);
 }