Add GitHub Action to test all supported Elixir versions

[krisalyssa]

GitHub Actions support build matrices, so it should be trivial [1] to support multiple runs for all of the supported versions of Elixir, at least to the minor version.

I enumerated through all the versions known to asdf from 1.6 on up, and came up with:

• 1.6.6-otp-19
• 1.6.6-otp-20
• 1.6.6-otp-21
• 1.7.4-otp-19
• 1.7.4-otp-20
• 1.7.4-otp-21
• 1.7.4-otp-22
• 1.8.2-otp-20
• 1.8.2-otp-21
• 1.8.2-otp-22
• 1.9.4-otp-20
• 1.9.4-otp-21
• 1.9.4-otp-22
• 1.10.3-otp-21
• 1.10.3-otp-22

[1] For varying values of "trivial".

[krisalyssa]

I would suggest that this is very low priority. I might tackle it in the next day or two, because I'm kind of in a GitHub Actions frame of mind currently.

[uesteibar]

Hey! This sounds great, but we already have this running on travis (https://travis-ci.org/github/uesteibar/neuron/builds), although we should add 1.10 too.

If you want to go ahead and do this using github actions, please feel free to go ahead, I haven't poked around those yet so I can learn something too ^^