Install dm_control from PyPI (bp #1247)

[mergify]

This is an automated backport of pull request #1247 done by Mergify

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[codecov]

Codecov Report

Merging #1406 into release-2019.10 will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@                 Coverage Diff                 @@
##           release-2019.10    #1406      +/-   ##
===================================================
- Coverage            85.66%   85.62%   -0.05%     
===================================================
  Files                  155      155              
  Lines                 7479     7479              
  Branches               928      928              
===================================================
- Hits                  6407     6404       -3     
- Misses                 894      896       +2     
- Partials               178      179       +1     

Impacted Files	Coverage Δ
src/garage/envs/dm_control/dm_control_env.py	72.54% <100.00%> (ø)
src/garage/envs/grid_world_env.py	84.12% <0.00%> (-4.77%)	⬇️

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