Reword README's language from third person to second person

Based on feedback in https://github.com/technicalpickles/homesick/pull/64, going with @christianbundy's:

> Your home directory is your castle. Don't leave your dotfiles behind.

I also noticed `Rakefile` had a reference to the language, so updated it there too. The language is still in `homesick.gemspec`, but that is managed by jeweler, and will get updated on next release anyways.