Enable testing with PostgreSQL 10 on PRs

[atodorov]

because kiwitcms-tenants needs PostgreSQL we enable testing with it for all PRs alongside MariaDB.

[codecov-io]

Codecov Report

Merging #895 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #895   +/-   ##
=======================================
  Coverage   73.52%   73.52%           
=======================================
  Files         108      108           
  Lines        4733     4733           
  Branches      605      605           
=======================================
  Hits         3480     3480           
+ Misses       1031     1030    -1     
- Partials      222      223    +1

Impacted Files	Coverage Δ
tcms/core/history.py	90.69% <0%> (ø)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 2952

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.187%

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Totals
Change from base Build 2941:	0.0%
Covered Lines:	3702
Relevant Lines:	4733

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💛 - Coveralls