Remove ext-xdebug check, add php-7.3 test

[peter279k]

Changed log

• It's related to issue #7.
• I think it's fine to remove ext-xdebug check because this extension is required when PHPUnit need to generate the coverage report. In Travis CI build, the phpunit command just do the unit test work, not generating code coverage report.
• It will be worked fine in the latest Travis CI build.
• According to this issue discussion thread, the php-nightly version is not loaded the XDebug extension in Travis CI build.
• Also, add the php-7.3 test during Travis CI build because this PHP version is stable.

[Lewiscowles1986]

I've integrated the line for php 7.3, but omitted the part removing x-debug. I would like this to automatically generate coverage stats