…/v0.14.12...v0.15.0)

**Closed issues:**
- Observing derived values via Turing.@addlogprob! are only handled by MH, not by IS, PG and SMC (#1467)
- observe function / @observe macro?  (#1471)

**Merged pull requests:**
- Update to AbstractMCMC 2 (#1428) (@devmotion)
- Fix random test errors (#1469) (@devmotion)
- Replace Travis with Github Action (#1470) (@devmotion)