Add `mixed` to the return type of `ObjectProphecy::reveal()`

[tjlytle]

PhpStorm complains about the type being wrong for methods with typed arguments because the annotated return value of reveal() is object and (I think) this plugin adds ProphecySubjectInterface. If mixed is added as well, PhpStorm should stop complaining that object|ProphecySubjectInterface don't match the expected type.