Ensure we supply Kernel#y for 1.9 too

In 1.9, it doesn't live in its own file, so we'll have to define it
ourselves.

Check RUBY_VERSION, instead of rescuing the require, because we want
this to break if `psych/y` moves in a future Ruby release.
Merge pull request #14597 from matthewd/kernel_y_for_1_9_option_1

Ensure we supply Kernel#y for 1.9 too