Remove deprecation warning in spec test

Remove the warning `DEPRECATION WARNING: `#deliver!` is deprecated and will be removed in Rails 5. Use `#deliver_now!` to deliver immediately or `#deliver_later!` to deliver through Active Job. (called from block (3 levels) in <top (required)> at /Users/dtaniwaki/github/mandriller/spec/support/action_mailer.rb:4)` in the spec test.
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