sebastianbergmann/phpunit#5033 changed phpunit
to no longer depend on phpspec/prophecy. This results in the need to add
a development dependency for phpspec/prophecy. Doing so results in a
warning, since PHPUnit\Framework\TestCase::prophesize() is deprecated
and will be removed in PHPUnit 10. Let's use the trait provided by
phpspec/prophecy-phpunit instead.

* Fixes myclabs#177

This reverts commit e03ad06.
Supporting phpspec/prophecy-phpunit at this point, would require
dropping php <= 7.2 support. We can re-visit this matter once we start
supporting phpunit 10.

While e03ad06 (reverted) is not applied, we require phpspec/prophecy as
a development dependency. This is needed because phpunit dropped
ptophecy as a dependency in recent releases. See e03ad06 for further
reference.