Update to latest RSpec.

The ~> 2.6.0 lock was introduced in
a5967cae81f6aa84e65ea865304314148698c534 due to the following issues:

  * https://github.com/rspec/rspec-core/issues/476
  * https://github.com/rspec/rspec-core/issues/477

Both issues appear to now be addressed, so let's get up to date.
Address RSpec deprecation warnings.