Use PHPUnit\Framework\TestCase instead of PHPUnit_Framework_TestCase

[samnela]

Build failed on unrelated reason , PHPUnit\Framework\TestCase already exists in PHPunit version 4.8.35

[peter279k]

@samnela, thank you for your reply.

According to this Travis CI build log, the errors are as follows:

• php-5.5 and php-5.5.9 have the problem about dependencies.
• php-5.6 and php-hhvm have the failed assertions.

[samnela]

Well, I think we should need another PR to remove php 5.5, 7.0 and all unmaintained version