Update version history
Update version history

- Mention 2018c-based release
- Mention 2.3.0-beta01

Fixes #1052.

malcolmr/nodatime,jskeet/nodatime,BenJenkinson/nodatime,malcolmr/nodatime,malcolmr/nodatime,nodatime/nodatime,nodatime/nodatime,jskeet/nodatime,BenJenkinson/nodatime,malcolmr/nodatime