Prefer $stderr over STDERR for consistency (#304)

Prior to this change there were only 3 instances of `STD(IN|OUT|ERR)`
versus 74 instances of `$std(in|out|err)` in the Homebrew code base. The
latter variant is also strongly suggested by bbatsov's Ruby Style Guide.

Library/Homebrew/formulary.rb

@@ -86,7 +86,7 @@ class Formulary
     private
 
     def load_file
-      STDERR.puts "#{$0} (#{self.class.name}): loading #{path}" if ARGV.debug?
+      $stderr.puts "#{$0} (#{self.class.name}): loading #{path}" if ARGV.debug?
       raise FormulaUnavailableError.new(name) unless path.file?
       Formulary.load_formula_from_path(name, path)
     end
@@ -199,7 +199,7 @@ class Formulary
     end
 
     def klass
-      STDERR.puts "#{$0} (#{self.class.name}): loading #{path}" if ARGV.debug?
+      $stderr.puts "#{$0} (#{self.class.name}): loading #{path}" if ARGV.debug?
       namespace = "FormulaNamespace#{Digest::MD5.hexdigest(contents)}"
       Formulary.load_formula(name, path, contents, namespace)
     end

Library/Homebrew/utils/popen.rb

@@ -20,7 +20,7 @@ module Utils
           return pipe.read
         end
       else
-        STDERR.reopen("/dev/null", "w")
+        $stderr.reopen("/dev/null", "w")
         exec(*args)
       end
     end