[CI] Upgrade to CentOS 7 + GCC 8

[hcho3]

• Upgrade to CentOS 7, since CentOS 6 has reached its end-of-life.
• Use Ninja to speed up builds.

Fixes recent failure in Linux build (https://dev.azure.com/hcho3/treelite/_build/results?buildId=679&view=logs&j=7536d2cd-87d4-54fe-4891-bfbbf2741d83&t=275bff32-624c-5245-5ea6-1890c77a0b18)

[codecov]

Codecov Report

Merging #313 (5432ab4) into mainline (c9766ca) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             mainline     #313    +/-   ##
============================================
  Coverage       85.18%   85.18%            
  Complexity         46       46            
============================================
  Files             108      108            
  Lines            8156     8156            
  Branches           50      466   +416     
============================================
  Hits             6948     6948            
  Misses           1183     1183            
  Partials           25       25            

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