VersionOne, Atlassian, Red Hat and MailJet as mixit14 sponsors

Not to be merged right now : wait for @fdepierre to create corporate profile of each one of theses companies. Follow-up doc : https://docs.google.com/document/d/1pQTriaQVSML3B6M1JiC_XgJHROWBv6ATYkGQEwkjtpc/edit