ImagineBlock - updated setImage typehint - fixes #123

Spotted by @wcluijt in https://github.com/symfony-cmf/BlockBundle/issues/123.

This PR solves 2 bugs:
- If the mediabundle is not used the typehint causes an error when doctrine generates the proxies;
- The typehint is also not correct as either an Uploaded file or ImageInterface can be set.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #123
| License       | MIT
| Doc PR        | N/A