update phpunit dep for expectException

[glensc]

this should fix travis errors that every PR in this project is suffering.

PHP Fatal error:
Call to undefined method
ZendTest\Mail\Protocol\SmtpPluginManagerCompatibilityTest::expectException()
in /home/travis/build/zendframework/zend-mail/vendor/zendframework/zend-servicemanager/src/Test/CommonPluginManagerTrait.php on line 56

this is because SmtpPluginManagerCompatibilityTest uses trait from other project CommonPluginManagerTrait which uses expectException phpunit method.

specified phpunit dependency that has such method available

[glensc]

probably broken since #72 got merged by @weierophinney, could you please merge this to master

EDIT: replaced 9494b07 commit with e776ea5

[weierophinney]

#132 updates the develop branch to PHPUnit 6, and does all of these changes already. Our next version will be 2.8.0, and incorporate those changes.

[glensc]

i hope you have look at all open PR's, i've submitted several ones. some are complete, some need feedback.