Test library with PHP 7.4

[markuspoerschke]

No description provided.

[peter279k]

The php-7.4 version is unstable.

If we want to use this preview version, I think we can use the - "7.4snapshot" in .travis.yml.

The useful references are as follows:

• PHPCompatibility/PHPCompatibility
• php-7.4 status in Travis
• Add PHP 7.4 branch to test against

[peter279k]

@markuspoerschke, the latest Travis CI build because of the php-cs-fixer.

The php-xs-fixer doesn't recognize the php-7.4-dev version.

[peter279k]

I think we can add the condition in Travis CI build setting:

If the PHP version is 7.4-dev, stopping the code coverage generating and coding style check.

And the PHPUnit cannot find the code coverage driver in php-7.4-dev during Travis CI build.

[ankurk91]

Php 7.4 stable is available on travis ci now