Hi Eugenio,
rcov isn't really required, the only thing it's used for is to check the test coverage, but since that's only of interest to someone actually writing tests, you won't need it.
Oh, and, I'd also like to warn you, Chameleon is not being very actively maintained anymore. As you may have seen, the latest release is already quite old, Chameleon still uses an old Ruby on Rails version, not all desired features have been implemented… I personally would be glad to help you with any problems you would encounter while trying to set up a blog using Chameleon, but I most likely won't be making any changes to the Chameleon code anymore. Of course, you can still make changes yourself, and I'd be glad to include them in the code.
Greetings,
JW