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 #30 (de96db8) into master (454e08d) will decrease coverage by 0.93%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
- Coverage   98.58%   97.65%   -0.94%     
==========================================
  Files           8        8              
  Lines         283      298      +15     
==========================================
+ Hits          279      291      +12     
- Misses          4        7       +3     

Impacted Files	Coverage Δ
src/townsend.jl	95.65% <0.00%> (-4.35%)	⬇️
src/gls.jl	96.87% <0.00%> (-3.13%)	⬇️
src/utils.jl	100.00% <0.00%> (ø)
src/extirpolation.jl	100.00% <0.00%> (ø)

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[giordano]

Thanks!