Use RSpec's `allow` syntax instead of `stub`

`allow` was added to RSpec 2 along with `expect` as part of the move
away from `should` and `stub` which monkey-patched `Object`. It will be
the default in RSpec 3.
Suggest a rebase/squash before pull requests

Performing a rebase/squash before a pull request performs an opportunity to
craft well-written commits before reviews are performed. This makes the review
easier, as the commits are easier to understand, and it allows review of the
actual commit titles and messages.