Which Salesforce License is required to use Dev Hub (beta)?

Salesforce plans to bring a new License type for this come Winter 18.

https://success.salesforce.com/_ui/core/chatter/groups/GroupProfilePage?fId=0D53A00003Dml6e&g=0F93A000000HTp1

We are working on providing a new type of free user license in the partner business org which will let you onboard developers in to the Dev Hub. Since the Dev Hub's primary purpose is to spin up scratch orgs, it can be shared across the different projects/customers you're working with. Alternatively, a customer can create a user for SI devs in their Dev Hub in which case you would use their scratch allocations.