Install error

[bosborne]

git clone git://github.com/Aissac/radiant-sitemap-xml-extension.git vendor/extensions/sitemap_xml
cd vendor/extensions
cd sitemap_xml
git checkout -b my_branch origin/0.9
Branch my_branch set up to track remote branch 0.9 from origin.
Switched to a new branch 'my_branch'

All those run without errors, then:

briano 1042 ~/ncg/vendor/extensions/sitemap_xml>rake radiant:extensions:sitemap_xml:migrate
(in /Users/briano/ncg/vendor/extensions/sitemap_xml)
rake aborted!
no such file to load -- spec/rake/spectask
/Users/briano/ncg/vendor/extensions/sitemap_xml/Rakefile:26