Drop support for Elixir < 1.10

[whatyouhide]

It's the oldest supported version by the Elixir team as of now.

[coveralls]

Pull Request Test Coverage Report for Build dadbe364be4f84aaf645df133462a1a77c2c59ee-PR-388

• 2 of 3 (66.67%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.08%) to 86.7%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/mint/core/transport/ssl.ex	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
lib/mint/core/transport/ssl.ex	2	82.17%

Totals
Change from base Build bb80a94afab40f8fd7cabdbb8d6a76f2580aa0f7:	-0.08%
Covered Lines:	1219
Relevant Lines:	1406

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💛 - Coveralls

[ericmj]

We need 1.5 support for Hex but if it that becomes a problem for Mint then it may be best if Hex forks it from before this commit. Are there features from 1.5+ we want to use in Mint?

[whatyouhide]

@ericmj oh, I completely missed that! There are a bunch, like is_struct/2 and stuff like that. I think we'll eventually have a hard time keeping support for as old as 1.5. I was thinking: should we keep the 1.4 release alive and have Hex use that instead? We can backport bug fixes if we deem necessary in the future.