Make installer script POSIX compliant

[profclems]

Make installer script POSIX compliant by removing bashisms and make it compatible with sh

Resolves #339

[codecov]

Codecov Report

Merging #612 (6208470) into master (644586b) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #612      +/-   ##
==========================================
+ Coverage   57.55%   57.60%   +0.05%     
==========================================
  Files          72       72              
  Lines        4446     4449       +3     
==========================================
+ Hits         2559     2563       +4     
  Misses       1644     1644              
+ Partials      243      242       -1     

Impacted Files	Coverage Δ
cmd/fork.go	53.84% <0.00%> (ø)
cmd/mr_create.go	75.38% <0.00%> (+0.90%)	⬆️

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[prarit]

This looks good to me. Thanks for the changes.