Remove actionmailer dependency

[ckhsponge]

Thanks for maintaining this!

I'd like the maintainers to consider removing actionmailer as a required dependency. Including actionmailer increases the size of our gem directory from 21 MB to 54 MB which is a large increase for something we don't need. Half of that increase is from nokogiri which is required by activesupport.

For the project I'm working on we are using the Slack notifier in a Sinatra app running on AWS Lambda. We're trying to keep the project light.

Changing it to a development dependency seems to work great for us:
ckhsponge@e3740c0

Let me know if you'd like a PR.

Thanks!