Stop testing with Julia 1.4 and 1.5

[tkf]

No description provided.

[codecov]

Codecov Report

Merging #514 (ca3dd46) into master (edbfb7c) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
+ Coverage   93.30%   93.38%   +0.08%     
==========================================
  Files          32       32              
  Lines        2224     2223       -1     
==========================================
+ Hits         2075     2076       +1     
+ Misses        149      147       -2     

Flag	Coverage Δ
Pkg.test	89.51% <ø> (+0.04%)	⬆️
Run.test	93.20% <ø> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
src/core.jl	89.71% <0.00%> (+0.41%)	⬆️
src/nondeterministic_threading.jl	89.79% <0.00%> (+1.02%)	⬆️

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