Define Symbol#to_proc for Ruby 1.8.6

Ruby 1.8.6 doesn't have Symbol#to_proc, which allows things like
map(&:to_s) rather than map { |o| o.to_s }. 1.8.7 does, though, and
since it is used in a bunch of the superenv code we should attempt to
keep it compatible with 1.8.6.

Closes #16046.