Remove PHPUnit as a development dependency

This will make it more inline with how Symfony uses PHPUnit and already
considers this a tool installed when developing.

for reference as PR that rejects having phpunit as dependency in symfony
https://github.com/symfony/symfony/issues/12136
minor #1039 Remove PHPUnit as a development dependency (henrikbjorn)

This PR was submitted for the master branch but it was merged into the 1.2 branch instead (closes #1039).

Discussion
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Remove PHPUnit as a development dependency

This will make it more inline with how Symfony uses PHPUnit and already
considers this a tool installed when developing.

for reference as PR that rejects having phpunit as dependency in symfony
https://github.com/symfony/symfony/issues/12136

Commits
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62b0fa7 Remove PHPUnit as a development dependency