Remove deprecation warning

- Using `should` from rspec-expectations' old `:should` syntax without
  explicitly enabling the syntax is deprecated. Use the new `:expect`
  syntax or explicitly enable `:should` instead.

- Calling #ago or #until on a number (e.g.  5.ago) is deprecated and
  will be removed in the future, use 5.seconds.ago instead.
Merge pull request #3 from akiinyo/remove-deprecation-warning

Remove deprecation warning