Test enhancement

[peter279k]

Changed log

• Using "vendor/bin/phpunit --coverage-clover=coverage.xml" to be composer test script.
• Add php-7.2 version test on Travis CI build.
• Using the --prefer-dist to install stable dependencies on composer install command executing.
• Removing the --dev option during composer install command executing because it's deprecated option.

The deprecated warning message is as follows:

You are using the deprecated option "dev". Dev packages are installed by default now.

[codecov]

Codecov Report

Merging #3 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             master     #3   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity       33     33           
=======================================
  Files             1      1           
  Lines            68     68           
=======================================
  Hits             68     68

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