Remove RSpec guideline for one-liners

Previously one-liners could be written in RSpec like:

    it { should validate_presence_of(:name) }

In RSpec 3, the `should` syntax was removed because it monkey-patched `Object`.
The one-liner syntax shown above was kept because its `should` did not
monkey-patch `Object`. To avoid confusion, RSpec 3 added an `expect`-style way
of writing one-liners:

    it { is_expected.to validate_presence_of(:name) }

According to Myron Marston:

> Some users have expressed confusion about how this should relates to the
> expect syntax and if you can continue using it. It will continue to be
> available in RSpec 3 (again, regardless of your syntax configuration), but
> we’ve also added an alternate API that is a bit more consistent with the
> expect syntax

From: [New API for
one-liners](http://myronmars.to/n/dev-blog/2014/05/notable-changes-in-rspec-3#new_api_for_oneliners_)

A proposal was made to switch to using the new `is_expected.to` syntaxin [this
pull request](https://github.com/thoughtbot/guides/pull/215). However, responses
were mixed. Given the lack of consensus one way or the other, let's remove the
guideline entirely and let each project decide which syntax to use.