Reverting to sprockets 3.7.2 re: segfault errors on ubuntu (SCP-3387)

[bistline]

This update downgrades the sprockets gem to 3.7.2 due to the known issue with segfaults when compiling static assets on ubuntu. There is no dependency in Rails 6 for using v4, so this downgrading should be fine moving forward, at least until they address the issue. This will help with both test stability and remove the chance that this issue will happen during a deployment.

This PR satisfies SCP-3387.

[codecov]

Codecov Report

Merging #1061 (2769276) into development (0bd4835) will increase coverage by 0.23%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development    #1061      +/-   ##
===============================================
+ Coverage        64.79%   65.02%   +0.23%     
===============================================
  Files              181      185       +4     
  Lines            16502    16525      +23     
  Branches           600      609       +9     
===============================================
+ Hits             10693    10746      +53     
+ Misses            5618     5585      -33     
- Partials           191      194       +3     

Impacted Files	Coverage Δ
lib/delayed_job_accessor.rb	88.88% <0.00%> (-5.56%)	⬇️
app/models/fire_cloud_client.rb	73.62% <0.00%> (-2.90%)	⬇️
...ollers/api/v1/visualization/clusters_controller.rb	85.88% <0.00%> (-2.86%)	⬇️
app/models/papi_client.rb	32.60% <0.00%> (-2.13%)	⬇️
app/models/big_query_client.rb	68.75% <0.00%> (-1.84%)	⬇️
app/models/parse_utils.rb	24.18% <0.00%> (-0.31%)	⬇️
app/lib/bulk_download_service.rb	86.66% <0.00%> (-0.09%)	⬇️
app/lib/user_asset_service.rb	94.59% <0.00%> (-0.08%)	⬇️
app/javascript/lib/scp-api-metrics.js	2.85% <0.00%> (ø)
...p/javascript/components/explore/plot-data-cache.js	93.75% <0.00%> (ø)
... and 33 more