boost: use Formula[]

Formula/boost.rb

@@ -115,7 +115,7 @@ class Boost < Formula
     bargs = ["--prefix=#{prefix}", "--libdir=#{lib}"]
 
     if build.with? 'icu'
-      icu4c_prefix = Formula.factory('icu4c').opt_prefix
+      icu4c_prefix = Formula['icu4c'].opt_prefix
       bargs << "--with-icu=#{icu4c_prefix}"
     else
       bargs << '--without-icu'
@@ -187,7 +187,7 @@ class Boost < Formula
       EOS
     end
 
-    if pour_bottle? and Formula.factory('python').installed?
+    if pour_bottle? and Formula['python'].installed?
       s += <<-EOS.undent
 
       The Boost bottle's module will not import into a Homebrew-installed Python.