Track the XML fixture in Git.

To make running benchmarks easier we'll track the XML file in Git in its
compressed form. I also decreased the size of the XML file from ~50 MB to
~10MB.

task/fixtures.rake

@@ -1,9 +1,6 @@
-desc 'Generates large XML fixtures'
-task :fixtures do
-  dest = File.expand_path('../../benchmark/fixtures/big.xml.gz', __FILE__)
-
-  unless File.file?(dest)
-    sh "wget http://downloads.yorickpeterse.com/files/big_xml_file.xml.gz -O #{dest}"
-    sh "gunzip #{dest}"
-  end
+rule '.xml' => '.xml.gz' do |task|
+  sh "gunzip #{task.source} --keep"
 end
+
+desc 'Generates large XML fixtures'
+task :fixtures => ['benchmark/fixtures/big.xml']