Previously, with nested arrays, the Symbol#to_proc would iterate over
the first item in the nested array instead of the array itself, e.g.:
[[1,2], [3,4]].map(&:first) #=>
NoMethodError: undefined method `first' for 1:Fixnum
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.
ClosesHomebrew/homebrew#16046.
