Upgrade to support both Citrus 2.4.x and 3.x

Hi!

I'm actually the one to blame because of the breaking change leading to Citrus 3.0 (a good reason, though). 

I saw the previous pull request on toml-rb and wanted to help with the transition from Citrus 2.4.x to 3.x. The pull request here shows how to keep a lib compatible with both Citrus 2.4.x and Citrus 3.x. I've updated the gemspec accordingly.

toml-rb tests succeed on my computer with both Citrus 2.4.1 and 3.0.1.