Release 2.3.0

- Added `ruby-1.9.3-p551`
- Added `ruby-2.0.0-p598`
- Added `ruby-2.1.5`
- Removed `ruby-1.9.3-p545`
- Removed `ruby-2.0.0-p481`
- Removed `ruby-2.1.2`
- Set `ruby-2.1.5` as default