Replace yarn calls with yarnpkg alias to prevent conflict with Hadoop YARN

[stanislas-m]

Fixes #1217.

Since yarnpkg/yarn#673, yarn defines an alias for itself: "yarnpkg". Its usage prevents conflicts with tools using the same name (such as Hadoop YARN, a common tool for Data scientists).

[codecov]

Codecov Report

Merging #1218 into development will not change coverage.
The diff coverage is 0%.

@@             Coverage Diff              @@
##           development    #1218   +/-   ##
============================================
  Coverage        55.16%   55.16%           
============================================
  Files               75       75           
  Lines             3865     3865           
============================================
  Hits              2132     2132           
  Misses            1603     1603           
  Partials           130      130

Impacted Files	Coverage Δ
buffalo/cmd/dev.go	4.76% <0%> (ø)	⬆️
buffalo/cmd/setup.go	4.69% <0%> (ø)	⬆️
buffalo/cmd/info.go	4% <0%> (ø)	⬆️

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