Test enhancement

[peter279k]

Changed log

• Set the php-nightly in allow_failures setting when executing the Travis build.
• Use the ::class for the \Exception class.
• Check the ext-mbstring when executing the Composer command.
• The composer self-update is executed by default in Travis build.
• The --dev option in composer command is deprecated.
• Use the composer update --prefer-dist -n to update the dependencies for the different PHP versions tests because the composer.lock will cache the dependencies version when executing the composer install.
• Add the autoload-dev to load the related Test.php automatically.

[coveralls]

Coverage remained the same at 100.0% when pulling a4b7892 on peter279k:test_enhancement into 5a882c8 on kornrunner:master.

[kornrunner]

Thank you!