[branch-0.9] manually update some version numbers to 0.9.3-SNAPSHOT

Author: Xiangrui Meng <meng@databricks.com>

Closes #1543 from mengxr/after-release-0.9.2 and squashes the following commits:

a74c730 [Xiangrui Meng] manually update some version numbers to 0.9.3-SNAPSHOT