I have installed GenemuFormBundle on my system, and after composer installed everything and tried to clear the cache, I got a LogicException stating that "Format png is not supported". After installing PHP's GD extension, everything worked fine.
This extension is not required as it's only used for captcha type AFAIK, you should manually disable it in order to fix this (cf. #181), it can probably be added under the suggest section but not require.
Anyway this won't be needed after the 2.2 version as all types will be disabled by default (#177 is pending).
[Captcha] Don't fail at compile-time when GD isn't available