Added shader support, removed unused debugging puts statements and fixed Gosu Font.draw deprecation warning.

shaders/fragment/lighting.glsl

@@ -0,0 +1,7 @@
+# version 150
+
+out vec4 frag_colour;
+
+void main() {
+  frag_colour = vec4(0.5, 0.0, 0.5, 1.0);
+}

shaders/vertex/lighting.glsl

@@ -0,0 +1,8 @@
+# version 150
+# extension GL_ARB_explicit_attrib_location : enable
+
+layout(location = 0) in vec3 vert;
+
+void main() {
+  gl_Position = vec4(vert, 1.0);
+}