Comment out `warn` used in the `Array#fetch` method
I get an error because the current mruby does not have a `Kernel#warn` method.
But the warning itself is useful and I'll just comment it out in case it's implemented in the future.
@@ -325,7 +325,7 @@ class Array
#
def fetch(n, ifnone=NONE, &block)
- warn "block supersedes default value argument" if !n.nil? && ifnone != NONE && block
+ #warn "block supersedes default value argument" if !n.nil? && ifnone != NONE && block
idx = n
if idx < 0
