Deprecation message from Imagine lib

[sdespont]

The library 2.13.3 has a deprecation message from Imagine when the filter is applied :

deprecation.INFO: User Deprecated: Method "Imagine\Filter\FilterInterface::apply()" might add "\Imagine\Image\ImageInterface" as a native return type declaration in the future. Do the same in implementation "Liip\ImagineBundle\Imagine\Filter\RelativeResize" now to avoid errors or add an explicit @return annotation to suppress this message.

[dbu]

we still support legacy PHP versions and therefor can't declare the return type on our implementations. i tagged this issue with the 3.0.0 milestone so we don't forget to clean this up when we create a new major version.

if the deprecation warnings annoy you, please do a pull request against the 2.x branch that adds the @return annotations to all filters.