Test enhancement

[peter279k]

Changed log

• Add php-7.2 and php-7.3 tests during Travis CI build.
• To be compatible with future PHPUnit versions, using the PHPUnit\Framework\TestCase class namesapce instead.
• satooshi/php-coveralls package is deprecated and using the php-coveralls/php-coveralls instead.
• Removing the hhvm version test because it will not be different from future PHP versions.

[coveralls]

Coverage decreased (-14.2%) to 83.019% when pulling dc4f911 on open-source-contributions:test_enhancement into e054418 on gpslab:master.

[coveralls]

Coverage decreased (-14.2%) to 83.019% when pulling dc4f911 on open-source-contributions:test_enhancement into e054418 on gpslab:master.

[coveralls]

Coverage decreased (-14.2%) to 83.019% when pulling dc4f911 on open-source-contributions:test_enhancement into e054418 on gpslab:master.

[coveralls]

Coverage decreased (-14.2%) to 83.019% when pulling dc4f911 on open-source-contributions:test_enhancement into e054418 on gpslab:master.

[coveralls]

Coverage decreased (-12.3%) to 84.906% when pulling 8bc2dcc on open-source-contributions:test_enhancement into e054418 on gpslab:master.

[coveralls]

Coverage decreased (-14.2%) to 83.019% when pulling dc4f911 on open-source-contributions:test_enhancement into e054418 on gpslab:master.

[peter279k]

@peter-gribanov, please look at this PR.

Thanks :).

[peter-gribanov]

Good job. Thanks.