My personal interpretation is that any project on GitHub that has no explicit license is covered under a very permissive BSD-style license (without any attribution requirements).
Typically a license grants you rights and informs you of your restrictions if you choose to exercise those rights. In this case, with no explicit attribution or GPL-like openness stated, but with forking a right implicit to public code on GitHub, I am led to believe I can take everything, then make closed modification to the software for my own personal or commercial use.
In other words - unless this is your intention in publishing on GitHub - you should definitely include a specific license.