Adds a deprecation warning that prints when you load the gem.

[philnash]

This adds a deprecation warning when you load the gem. I was going to release a new version (3.0.1) of the gem so that RubyGems can pick up the new description and summary, so I thought, while I'm at it, why not a deprecation message for those that update their gems.

@robinske do you think this is a good idea?

Contributing to Twilio

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[philnash]

I can archive the repo once this is decided.

[robinske]

Yeah I think it's a great idea - thanks for doing the new release.