[ruby/zlib] Bump up zlib version to 2.1.0

https://github.com/ruby/zlib/commit/dd593acaee

ext/zlib/zlib.c

@@ -25,7 +25,7 @@
 # define VALGRIND_MAKE_MEM_UNDEFINED(p, n) 0
 #endif
 
-#define RUBY_ZLIB_VERSION "2.0.0"
+#define RUBY_ZLIB_VERSION "2.1.0"
 
 #ifndef RB_PASS_CALLED_KEYWORDS
 # define rb_class_new_instance_kw(argc, argv, klass, kw_splat) rb_class_new_instance(argc, argv, klass)