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 Homebrew/homebrew#16046.

Library/Homebrew/extend/symbol.rb

@@ -0,0 +1,5 @@
+class Symbol
+  def to_proc
+    proc { |obj, *args| obj.send(self, *args) }
+  end unless method_defined?(:to_proc)
+end

Library/Homebrew/global.rb

@@ -2,6 +2,7 @@ require 'extend/fileutils'
 require 'extend/pathname'
 require 'extend/ARGV'
 require 'extend/string'
+require 'extend/symbol'
 require 'utils'
 require 'exceptions'
 require 'set'