Use compact form of head where possible

Formula/djvulibre.rb

@@ -18,9 +18,7 @@ class Djvulibre < Formula
     sha1 "96a3c4bd45ed2c2ca4a6efab0f1334326560b891" => :lion
   end
 
-  head do
-    url 'git://git.code.sf.net/p/djvu/djvulibre-git'
-  end
+  head 'git://git.code.sf.net/p/djvu/djvulibre-git'
 
   depends_on 'jpeg'
   depends_on 'libtiff'

Formula/mu.rb

@@ -16,9 +16,7 @@ class Mu < Formula
   url 'https://github.com/djcb/mu/archive/v0.9.9.6.tar.gz'
   sha1 '45feb511fecf8b306c87a42f3a84858ac442642c'
 
-  head do
-    url 'https://github.com/djcb/mu.git'
-  end
+  head 'https://github.com/djcb/mu.git'
 
   option 'with-emacs', 'Build with emacs support'
 

Formula/phantomjs.rb

@@ -21,9 +21,7 @@ class Phantomjs < Formula
     sha1 "273dbe33d1edbdd034c903d919278d33d7ebe5dd" => :lion
   end
 
-  head do
-    url "https://github.com/ariya/phantomjs.git"
-  end
+  head "https://github.com/ariya/phantomjs.git"
 
   def install
     if build.stable? && MacOS.prefer_64_bit?