update mailgun.cr

[vectorselector]

Updated mailgun.cr for Crystal 0.31.1 per Crystal API breaking change crystal-lang/crystal#7085

[crisward]

@vectorselector Thanks for taking the time to submit these pull requests.

I'm not actively using crystal at the moment, but have some projects which rely on this repo and and older version of crystal that this was programmed to work with.

So although I'll be happy to merge these changes in, it will probably won't be a for a while.

Thanks again.

[vectorselector]

Oh, I did that with a couple projects pegged at Crystal 0.25.1 via crenv
I usually then pull the repo local under the project directory as such:
mailer: path: crystal_libraries/mailer

vs

mailer: github: crisward/mailer

[spicybackend]

Thanks heaps for the PRs @vectorselector! Also taking on your dependency suggestion here and it's working wonders. It'd be awesome to get these changes merged and released sometime soon though!