Test enhancement

[peter279k]

Changed log

• Set multiple PHPUnit version to support multiple PHP versions tests.
• Use the class-based PHPUnit namespace and expected exception annotation to be compatible with the stable PHPUnit version.
• Add php-7.2 test in Travis build.
• According to the PHPUnit version in require-dev, it set the 5.0 and it supports the php-5.6+. I set the PHP version >=5.6 in require .

[coveralls]

Coverage remained the same at 99.652% when pulling bc15ccd on peter279k:test_enhancement into 3cc5ae4 on mrjgreen:master.

[coveralls]

Coverage remained the same at 99.652% when pulling bc15ccd on peter279k:test_enhancement into 3cc5ae4 on mrjgreen:master.