PHPunit 6 update

[kokspflanze]

• phpunit PHPUnit_Framework_TestCase migration to namespace
• bootstrap removed phpunit 4 case
• setExpectedException changed to expectException
• drop php5.5
• update phpunit version
• composer.lock added
• drop hhvm from build matrix

[weierophinney]

While you're at it, set the fast_finish flag to false, please. :)

[kokspflanze]

i removed it already =) with https://github.com/zendframework/zend-mime/pull/28/commits/3a025b2ea98d24d6402e56a15e34434422800183

i missed it=)

[weierophinney]

Thanks, @kokspflanze!