Enhancement: Add PHP 7.3 to Travis CI build matrix

[localheinz]

This PR

• adds PHP 7.3 to the Travis CI build matrix
• removes localheinz/php-cs-fixer-config on PHP 7.3

[codecov]

Codecov Report

Merging #115 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             master   #115   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity      136    136           
=======================================
  Files            30     30           
  Lines           415    415           
=======================================
  Hits            415    415

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