Upgrade rebar3 to a safe version

[kostis]

Closes #277

[codecov-commenter]

Codecov Report

Merging #278 (3703c8e) into master (1daf130) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage   85.52%   85.56%   +0.04%     
==========================================
  Files          14       14              
  Lines        4586     4586              
==========================================
+ Hits         3922     3924       +2     
+ Misses        664      662       -2     

Impacted Files	Coverage Δ
src/proper_statem.erl	94.67% <0.00%> (-0.39%)	⬇️
src/proper_types.erl	95.09% <0.00%> (ø)
src/proper_erlang_abstract_code.erl	94.40% <0.00%> (+0.16%)	⬆️
src/proper_arith.erl	92.70% <0.00%> (+1.04%)	⬆️

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