Remove deprecation warning

以下2つの deprecation warning に対応してみました :santa: 

- 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.