[chef] knife-ec2 plugin not working when chef is installed from Ubuntu packages

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Hi,

I've installed chef from the opscode Ubuntu packages. I have the version
0.10.2-2.
I've tried to install knife-ec2 from the rubygems packages installation.
Rubygems version is 1.3.7.
If I try to use 'knife ec2 ....' it keeps saying:
FATAL: Cannot find sub command for: 'ec2'
The ec2 commands were moved to plugins in Chef 0.10
You can install the plugin with `(sudo) gem install knife-ec2

Any ideas?

Thanks
--
Luis