Support ChainRulesCore v1

[oxinabox]

It's nice that this didn't break anything here.
It would be nice to get this merged relatively quickly, as SpecialFunctions is depended on by many packages and thus will block the ChainRulesCore v1 rollout

[codecov]

Codecov Report

Merging #335 (f8df1ba) into master (42bb273) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #335      +/-   ##
==========================================
- Coverage   92.61%   92.49%   -0.12%     
==========================================
  Files          12       12              
  Lines        2653     2653              
==========================================
- Hits         2457     2454       -3     
- Misses        196      199       +3     

Flag	Coverage Δ
unittests	92.49% <ø> (-0.12%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/beta_inc.jl	89.61% <0.00%> (-0.47%)	⬇️

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