Parallelize unit tests on CI server (Travis)

[DonJayamanne]

Fixes #1247
Temporary work around for #1250 (didn't add a news entry for the work around)

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[codecov]

Codecov Report

Merging #1251 into master will increase coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1251      +/-   ##
==========================================
+ Coverage   70.94%   71.11%   +0.17%     
==========================================
  Files         266      266              
  Lines       12284    12284              
  Branches     2172     2172              
==========================================
+ Hits         8715     8736      +21     
+ Misses       3445     3417      -28     
- Partials      124      131       +7

Impacted Files	Coverage Δ
src/client/linters/lintingEngine.ts	90.35% <0%> (-0.88%)	⬇️
...rc/client/debugger/PythonProcessCallbackHandler.ts	53.61% <0%> (+0.65%)	⬆️
src/client/debugger/Main.ts	51.85% <0%> (+1.23%)	⬆️
src/client/debugger/PythonProcess.ts	51.25% <0%> (+2.5%)	⬆️
src/client/debugger/mainV2.ts	75.67% <0%> (+4.05%)	⬆️

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