Train::Extras::CommandWrapper::LinuxCommand#verify needs to be cleaned up

As pointed out in #508, so many things wrong:

* This method has a return value of `nil | String | [String, Symbol]`. No, nil or String only.
* The `each` inside should be cleaned up (to at least break when found).
* The block argument name `sudo` is bad.
* Formatting is inconsistent.
* The name of the class itself seems to be misleading (any unix, not just linux?).