As travis-rubies now uses 3 Mac jobs to create archives for various OS
releases.

This is a bit wasteful if multiple builds pass in a short period.

Instead, Travis CI is now running a nightly build of mruby-head
(around 22:30 UTC). This will happen regardless of the state of the
master branch.

Correct argument specifications for few methods

 - Struct#values_at
 - Module#define_method
 - String#chop
 - String#chop!

Fixed by Alex Snaps and reported by Mathieu Leduc-Hamel,
both from shopify.com.  Thank you!

Reported from from Alex Snaps via Mathieu Leduc-Hamel,
both from shopify.com.  Thank you!

Reported from Alex Snaps via Mathieu Leduc-Hamel,
both from shopify.com.  Thank you!

Class#to_s used to return same string repeatedly,
that mean you can modify "class name" by modifying
the return value from Class#to_s.

Solves #2132.

Module#define_method supports proc argument

Reimplement Enumerable#cycle 

Fix pattern of infinite loop
And support all specs in https://github.com/ruby/spec/blob/27960d06e0ce92c37f074450f0eab4b0519b118c/core/enumerable/cycle_spec.rb without Enumerable#size

test/assert.rb should not use puts

Add mrb_class_defined_under

Update documentation of fetch

The sentence `Negative values of +index+ count from the end of the array.` can be interpreted that it only holds if a block is given. Clarify it.

Kernel#respond_to? should return true|false only