oop, rank was not allowing index == 0

the desc for the index param to rank was incorrectly banning index == 0

see https://github.com/jcupitt/libvips/issues/555

libvips/morphology/rank.c

@@ -24,6 +24,8 @@
  * 	- gtk-doc
  * 17/1/14
  * 	- redone as a class
+ * 12/11/16
+ * 	- oop, allow index == 0, thanks Rob
  */
 
 /*
@@ -430,7 +432,7 @@ vips_rank_class_init( VipsRankClass *class )
 		_( "Select pixel at index" ),
 		VIPS_ARGUMENT_REQUIRED_INPUT,
 		G_STRUCT_OFFSET( VipsRank, index ),
-		1, 100000000, 50 );
+		0, 100000000, 50 );
 
 }