Update DataStructures compat to include 0.19

[cake-duke]

DataStructures.jl has some breaking changes[1]. However they don't affect this code.

https://github.com/JuliaCollections/DataStructures.jl/releases/tag/v0.19.0

[cake-duke]

This #45 fixed the tests, please re-run.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.44%. Comparing base (37d701f) to head (760b792).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
- Coverage   52.27%   49.44%   -2.83%     
==========================================
  Files          12       12              
  Lines         461      453       -8     
==========================================
- Hits          241      224      -17     
- Misses        220      229       +9     

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