Fix required CMake version

[Flamefire]

cxx_std_11 requires CMake 3.8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.85%. Comparing base (4f9e990) to head (2749119).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #139   +/-   ##
========================================
  Coverage    95.85%   95.85%           
========================================
  Files          115      115           
  Lines         8128     8128           
========================================
  Hits          7791     7791           
  Misses         337      337           

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[mborland]

LGTM Thank you!