SPDX 3.0 support

[pmonks]

SPDX v3.0 is currently in RC and a GA will likely be available soon, and while it mostly doesn't impact clj-spdx (being mostly model changes, which this library doesn't yet implement), there will be some changes necessary, including:

• Waiting for SPDX 3.0 Support spdx/Spdx-Java-Library#178 (note: technically optional, given we don't use the model classes from that library that are most affected by the new version of the spec)
• Update the SPDX expression parser to support AdditionRefs (done in rev e75f9cd)

[pmonks]

Released in 1.0.174, without upgrading Spdx-Java-Library. That can happen later, whenever it's ready.