Require "rspec/core" not "rspec".

There are cases where rspec-core is present but rspec is not.

The rspec-rails gem as of 2.12.0 does not depend on "rspec", but instead
lists "rspec-core" and other rspec gems as dependencies.
Merge pull request #12 from pda/require-rspec-core

Require "rspec/core" not "rspec".