Specified clang-tidy version 8 for contributing guidelines

[mathfac]

Motivation and Context

Specified clang-tidy version 8 for contributing guidelines, as our clang config isn't backward compatible.

How Has This Been Tested

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[codecov-io]

Codecov Report

Merging #408 into master will decrease coverage by 0.78%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #408      +/-   ##
==========================================
- Coverage   56.74%   55.96%   -0.79%     
==========================================
  Files         175      171       -4     
  Lines        8094     7950     -144     
  Branches       84       84              
==========================================
- Hits         4593     4449     -144     
  Misses       3501     3501

Flag	Coverage Δ
#CPP	52.35% <ø> (ø)	⬆️
#JavaScript	10% <ø> (ø)	⬆️
#Python	77.42% <ø> (-1.41%)	⬇️

Impacted Files	Coverage Δ
tests/test_sensors.py	100% <0%> (ø)	⬆️
habitat_sim/sensors/noise_models/__init__.py	100% <0%> (ø)	⬆️
...t_sim/sensors/noise_models/gaussian_noise_model.py
...ensors/noise_models/salt_and_pepper_noise_model.py
...at_sim/sensors/noise_models/speckle_noise_model.py
...at_sim/sensors/noise_models/poisson_noise_model.py

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