OTP24

[josefs]

Use OTP 24 in travis and use native implementation of maps:merge_with when available.

[josefs]

Travis doesn't support the latest version of OTP yet. I didn't check before submitting the PR. Oh well, I guess I'll just leave this one here for the time being.

[codecov-commenter]

Codecov Report

Merging #335 (8500530) into master (e226803) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #335   +/-   ##
=======================================
  Coverage   87.46%   87.46%           
=======================================
  Files          18       18           
  Lines        2760     2760           
=======================================
  Hits         2414     2414           
  Misses        346      346           

Impacted Files	Coverage Δ
src/gradualizer_lib.erl	88.17% <ø> (ø)

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