Moved dependencies to build_value 5

[charafau]

Due to some changes in dart version in flutter developers needs to increase also build_value version to 5.

It also fixes this: #15

[codecov-io]

Codecov Report

Merging #16 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #16   +/-   ##
=======================================
  Coverage   96.15%   96.15%           
=======================================
  Files           1        1           
  Lines          26       26           
=======================================
  Hits           25       25           
  Misses          1        1

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[davidmarne]

thanks!