Use expect syntax

According to http://betterspecs.org, RSpec suites should use the newer `expect` syntax over the older `should` syntax, and example descriptions should not start with the word "should". This pull request makes homesick's specs follow these recommendations while disabling RSpec's older `should` syntax.

I used [transpec](http://yujinakayama.me/transpec/) and [should_clean](https://github.com/siyelo/should_clean) to help fix these issues (though I have reviewed the resulting changes).