Release 2.1.0

• Minimum Ruby version now 2.6.
• Update doap:license to https://unlicense.org/1.0/.
• Uses Linkeddata 3.2.

Release 2.0.1

• Remove support for rack < 2.0, as rack-linkeddata now requires this.
• Updates for RDF 3.1 release and Ruby 2.7 calling sequences.