[AllBundles] Fix fixture deprecations

[dannyvw]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	yes
Fixed tickets	comma separated list of tickets fixed by the PR

Method "Doctrine\Common\DataFixtures\FixtureInterface::load()" might add "void" as a native return type declaration in the future. Do the same in implementation "Kunstmaan\GeneratorBundle\DataFixtures\ORM\GroupFixtures" now to avoid errors or add an explicit @return annotation to suppress this message.

[acrobat]

Technically adding those return types are a BC break, but I don't expect anyone extending the fixture classes so let's merge as is! Thanks @dannyvw!