Fetch Version Fix

[leonardocustodio]

This fixes the fetch version to get the latest mobx versions but ignores pre-release versions.

[codecov]

Codecov Report

Merging #623 (425b043) into master (0404d63) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #623   +/-   ##
=======================================
  Coverage   84.07%   84.07%           
=======================================
  Files          52       52           
  Lines        1796     1796           
=======================================
  Hits         1510     1510           
  Misses        286      286           

Flag	Coverage Δ
flutter_mobx	100.00% <ø> (ø)
mobx	83.45% <ø> (ø)
mobx_codegen	30.20% <ø> (ø)

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