Fix hard dependency on gd

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #71 
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/issues/302