Improve tests and move to Github Actions

[denisvmedia]

• Remove Travis CI support.
• Add Github Actions support.
• Add integration Tests with Symfony 4.4 and 5.1.
• Upgrade various dependencies (minimal supported Symfony version is 4.4).
• Improve tests by fixing deprecations.

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (develop@534a881). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop      #20   +/-   ##
==========================================
  Coverage           ?   95.00%           
  Complexity         ?       78           
==========================================
  Files              ?       11           
  Lines              ?      220           
  Branches           ?        0           
==========================================
  Hits               ?      209           
  Misses             ?       11           
  Partials           ?        0           

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