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 for non-integer arguments(?) this shouldn't affect you.

[Datseris]

Thank you very much for opening this, which reminded me that I need to set up CompatHelper for this repo! :)

[codecov]

Codecov Report

Merging #62 (70e46ac) into master (4d94d76) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #62   +/-   ##
=======================================
  Coverage   78.40%   78.40%           
=======================================
  Files          19       19           
  Lines         565      565           
=======================================
  Hits          443      443           
  Misses        122      122           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4d94d76...70e46ac. Read the comment docs.