Change doc version to 2.8-dev
Change doc version to 2.8-dev

It's shown in the front page of readthedocs/latest.

markalanj/jansson,Mephistophiles/jansson,akheron/jansson,bstarynk/jansson,akheron/jansson,wirebirdlabs/featherweight-jansson,npmccallum/jansson,Mephistophiles/jansson,bstarynk/jansson,npmccallum/jansson,bstarynk/jansson,AmesianX/jansson,wirebirdlabs/featherweight-jansson,markalanj/jansson,akheron/jansson,AmesianX/jansson,markalanj/jansson