Remove backports for Ruby 1.8.6

refs. Homebrew/linuxbrew#438, mistydemeo/tigerbrew#327,
mistydemeo/tigerbrew#334

Library/Homebrew/extend/enumerable.rb

-module Enumerable
-  def group_by
-    inject({}) do |h, e|
-      h.fetch(yield(e)) { |k| h[k] = [] } << e; h
-    end
-  end unless method_defined?(:group_by)
-end

Library/Homebrew/extend/string.rb

@@ -15,33 +15,6 @@ class String
   #               EOS
   alias_method :undent_________________________________________________________72, :undent
 
-  def start_with?(*prefixes)
-    prefixes.any? do |prefix|
-      if prefix.respond_to?(:to_str)
-        prefix = prefix.to_str
-        self[0, prefix.length] == prefix
-      end
-    end
-  end unless method_defined?(:start_with?)
-
-  def end_with?(*suffixes)
-    suffixes.any? do |suffix|
-      if suffix.respond_to?(:to_str)
-        suffix = suffix.to_str
-        self[-suffix.length, suffix.length] == suffix
-      end
-    end
-  end unless method_defined?(:end_with?)
-
-  # 1.8.7 or later; used in bottle code
-  def rpartition(separator)
-    if ind = rindex(separator)
-      [slice(0, ind), separator, slice(ind+1, -1) || '']
-    else
-      ['', '', dup]
-    end
-  end unless method_defined?(:rpartition)
-
   # String.chomp, but if result is empty: returns nil instead.
   # Allows `chuzzle || foo` short-circuits.
   def chuzzle

Library/Homebrew/extend/symbol.rb

-class Symbol
-  def to_proc
-    proc { |*args| args.shift.send(self, *args) }
-  end unless method_defined?(:to_proc)
-end

Library/Homebrew/global.rb

@@ -3,8 +3,6 @@ require 'extend/fileutils'
 require 'extend/pathname'
 require 'extend/ARGV'
 require 'extend/string'
-require 'extend/symbol'
-require 'extend/enumerable'
 require 'os'
 require 'utils'
 require 'exceptions'