[CI] Downgrade libomp package to 11.1.0

[hcho3]

https://app.travis-ci.com/github/dmlc/treelite/jobs/524580900#L3132:

tests/python/test_basic.py::test_basic[mushroom-True-None-True-gcc] 
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong
with the build itself.
Check the details on how to adjust your build configuration on:
https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

I suspect it has to do with the use of OpenMP in src/annotator.cc.

[codecov]

Codecov Report

Merging #299 (55e3e46) into mainline (b2f52bd) will increase coverage by 0.23%.
The diff coverage is n/a.

❗ Current head 55e3e46 differs from pull request most recent head 5a5577c. Consider uploading reports for the commit 5a5577c to get more accurate results

@@              Coverage Diff               @@
##             mainline     #299      +/-   ##
==============================================
+ Coverage       84.75%   84.98%   +0.23%     
  Complexity         46       46              
==============================================
  Files             104      106       +2     
  Lines            7019     8040    +1021     
  Branches           50      461     +411     
==============================================
+ Hits             5949     6833     +884     
- Misses           1045     1182     +137     
  Partials           25       25              

Impacted Files	Coverage Δ
src/compiler/native/pred_transform.h	80.82% <0.00%> (-11.64%)	⬇️
include/treelite/frontend.h	90.00% <0.00%> (-10.00%)	⬇️
src/filesystem.cc	50.00% <0.00%> (-7.15%)	⬇️
src/gtil/pred_transform.cc	94.25% <0.00%> (-5.75%)	⬇️
include/treelite/predictor.h	70.73% <0.00%> (-5.03%)	⬇️
src/compiler/elf/elf_formatter.cc	96.07% <0.00%> (-3.93%)	⬇️
src/data.cc	80.76% <0.00%> (-3.51%)	⬇️
src/json_serializer.cc	92.17% <0.00%> (-2.33%)	⬇️
src/frontend/builder.cc	75.47% <0.00%> (-2.31%)	⬇️
src/frontend/lightgbm.cc	85.43% <0.00%> (-2.00%)	⬇️
... and 53 more

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[hcho3]

Indeed, downgrading libomp to 11.1.0 fixes the tests. It's the same issue as dmlc/xgboost#7039. For now, downgrade libomp to 11.1.0 like dmlc/xgboost#7107.