Test enhancement

[peter279k]

Changed log

• The php-7.4 version should be successful on Travis CI build.
• Using the --prefer-dist to install stable dependencies during composer install command executing.
• Removing $formatter variable because it's unused.

[codecov]

Codecov Report

Merging #9 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master       #9   +/-   ##
=========================================
  Coverage     98.55%   98.55%           
  Complexity       31       31           
=========================================
  Files             3        3           
  Lines            69       69           
=========================================
  Hits             68       68           
  Misses            1        1

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[Ayesh]

Another good one, thank you!