Update Travis environment to Ubuntu 18.04

[andreyv]

See https://docs.travis-ci.com/user/reference/bionic.

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This change is 

[codecov]

Codecov Report

Merging #1161 into master will decrease coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1161      +/-   ##
==========================================
- Coverage   61.99%   61.78%   -0.22%     
==========================================
  Files         230      230              
  Lines       16604    16604              
==========================================
- Hits        10294    10259      -35     
- Misses       6310     6345      +35     

Flag	Coverage Δ
#functionaltests	45.46% <ø> (-0.14%)	⬇️
#unittests	43.29% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cms/grading/Job.py	78.19% <0.00%> (-10.91%)	⬇️
cms/server/contest/handlers/tasksubmission.py	43.10% <0.00%> (-3.45%)	⬇️
cms/server/admin/handlers/task.py	39.84% <0.00%> (-1.18%)	⬇️
cms/db/base.py	82.97% <0.00%> (-1.07%)	⬇️
cms/service/ProxyService.py	57.97% <0.00%> (-1.07%)	⬇️
cms/service/esoperations.py	78.01% <0.00%> (-0.71%)	⬇️
cms/server/admin/handlers/base.py	67.11% <0.00%> (-0.68%)	⬇️
cms/io/service.py	68.90% <0.00%> (-0.61%)	⬇️
cms/server/admin/handlers/dataset.py	25.38% <0.00%> (-0.31%)	⬇️
cms/service/workerpool.py	61.66% <0.00%> (ø)
... and 4 more

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[andreyv]

Travis CI result:

2020-09-25 14:52:30,001 - INFO [<unknown>] Failed tests:
2020-09-25 14:52:30,001 - INFO [<unknown>] timeout-cputime (Haskell / ghc): Should have timed out, got Execution killed (could be triggered by violating memory limits)

Will need to investigate.

Also integration with GitHub seems to be broken.

[wil93]

I opened PR #1162 with a fix by @Mark03, who originally wrote the test. Hopefully it will fix the Travis build 😄

[andreyv]

Thanks! :)

I guess last [0 ..] now may run faster and fill memory before timing out?

EDIT: Although I see that the trusty build in #1162 also failed, so it may be that Travis made some changes to their environment recently.