Update Ubuntu image on CI to use 18.04

[atifaziz]

This PR updates the Ubuntu image used on CI to use 18.04.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d2b0a3f) 92.11% compared to head (94097c4) 92.11%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1057   +/-   ##
=======================================
  Coverage   92.11%   92.11%           
=======================================
  Files         112      112           
  Lines        3372     3372           
  Branches     1051     1051           
=======================================
  Hits         3106     3106           
  Misses        199      199           
  Partials       67       67           

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