build dependencies before building the app

[martinbonnin]

Fixes #429

[codecov-io]

Codecov Report

Merging #430 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master    #430   +/-   ##
========================================
  Coverage      97.8%   97.8%           
  Complexity      325     325           
========================================
  Files           105     105           
  Lines          1230    1230           
  Branches        202     202           
========================================
  Hits           1203    1203           
  Misses            7       7           
  Partials         20      20

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[smyrick]

I updated the docs in this PR with a little more detail: #431