libvorbis: use spec deps

Formula/libvorbis.rb

@@ -5,7 +5,13 @@ class Libvorbis < Formula
   url 'http://downloads.xiph.org/releases/vorbis/libvorbis-1.3.3.tar.xz'
   sha1 '31d1a0ec4815bf1ee638b0f2850f03efcd48022a'
 
-  head 'http://svn.xiph.org/trunk/vorbis'
+  head do
+    url 'http://svn.xiph.org/trunk/vorbis'
+
+    depends_on :autoconf
+    depends_on :automake
+    depends_on :libtool
+  end
 
   option :universal
 
@@ -13,12 +19,6 @@ class Libvorbis < Formula
   depends_on 'pkg-config' => :build
   depends_on 'libogg'
 
-  if build.head?
-    depends_on :autoconf
-    depends_on :automake
-    depends_on :libtool
-  end
-
   def install
     ENV.universal_binary if build.universal?