Downgrade coverage to workaround bug

[AdamGleave]

CI tests are triggering bug coveragepy/coveragepy#881 see e.g. https://circleci.com/gh/HumanCompatibleAI/imitation/193#queue-placeholder/containers/1 and https://circleci.com/gh/HumanCompatibleAI/imitation/185#tests/containers/1

Downgrade to 4.5.4 which does not suffer from this bug.

[codecov]

Codecov Report

Merging #140 into master will increase coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
+ Coverage   85.27%   85.45%   +0.17%     
==========================================
  Files          59       59              
  Lines        4001     4001              
==========================================
+ Hits         3412     3419       +7     
+ Misses        589      582       -7

Impacted Files	Coverage Δ
src/imitation/scripts/train_adversarial.py	98.62% <0%> (+2.06%)	⬆️
src/imitation/scripts/config/train_adversarial.py	64.89% <0%> (+2.12%)	⬆️
src/imitation/scripts/expert_demos.py	87.34% <0%> (+2.53%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update baa8e1c...2c06046. Read the comment docs.