Upgrade wp-cli to version 0.4.0.
Adjust the location of the source files because the directory
structure has changed.
Use `wp-cli.git', :tag => 'v0.4.0'` as the url so we checkout the
submodule too.
Add a version string to help with the odd tag name.
ClosesHomebrew/homebrew#12853.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>