Prefix version tags with "v".

This makes them stand out as versions a bit more.

task/tag.rake

@@ -2,5 +2,5 @@ desc 'Creates a Git tag for the current version'
 task :tag do
   version = Oga::VERSION
 
-  sh %Q{git tag -a -m "Version #{version}" #{version}}
+  sh %Q{git tag -a -m "Version #{version}" v#{version}}
 end