support specialfunctions 2.x

[stevengj]

The only breaking change in SpecialFunctions 2.0 was deleting the factorial(x::Real) = gamma(x+1) function, and since you don't use factorial this shouldn't affect you.

[codecov]

Codecov Report

Merging #35 (c0d5839) into master (b382d12) will decrease coverage by 1.87%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   90.27%   88.40%   -1.88%     
==========================================
  Files           9        9              
  Lines         360      388      +28     
==========================================
+ Hits          325      343      +18     
- Misses         35       45      +10     

Flag	Coverage Δ
unittests	88.40% <ø> (-1.88%)	⬇️

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

Impacted Files	Coverage Δ
src/scaling.jl	88.67% <0.00%> (-7.24%)	⬇️
src/morphology.jl	97.53% <0.00%> (-2.47%)	⬇️
src/inject.jl	89.79% <0.00%> (-2.16%)	⬇️
src/geometry/geometry.jl	100.00% <0.00%> (ø)
src/geometry/annulus.jl	96.87% <0.00%> (+0.10%)	⬆️
src/geometry/multiannulus.jl	96.87% <0.00%> (+0.10%)	⬆️
src/masking.jl	46.66% <0.00%> (+0.15%)	⬆️

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