Only check @specs for :using if @specs exists. Fixes Homebrew/homebrew#1127.

Library/Homebrew/formula.rb

@@ -134,6 +134,7 @@ class Formula
   
   # reimplement if we don't autodetect the download strategy you require
   def download_strategy
+    if @specs
       vcs = @specs.delete :using
       if vcs != nil
         # If a class is passed, assume it is a download strategy
@@ -150,6 +151,7 @@ class Formula
 
         raise "Unknown strategy #{vcs} was requested."
       end
+    end
 
     detect_download_strategy url
   end