Add init-style suites for installation from source

There’s really no point in having a `source` suite on it’s own. Much
better to test that Kafka actually starts when installed from source
rather than files just existing. Regressions are a bitch, man.
Update Rake task to ignore `gemfiles` directory
Tidy up Rakefile

* Removed part of a commented line.
* Clarified “error” message.