remove superfluous includes

- reduce compile time by a little bit (full-core: ~0.7s for me)
- thanks to 'include-what-you-use' for some help
- include Standard C header files before any other (coding style)

src/array.c

@@ -4,7 +4,6 @@
 ** See Copyright Notice in mruby.h
 */
 
-#include <limits.h>
 #include "mruby.h"
 #include "mruby/array.h"
 #include "mruby/class.h"