[branch-0.9] bump versions for v0.9.2 release candidate

Manually update some version numbers.

Author: Xiangrui Meng <meng@databricks.com>

Closes #1458 from mengxr/v0.9.2-rc and squashes the following commits:

2c38419 [Xiangrui Meng] Merge remote-tracking branch 'apache/branch-0.9' into v0.9.2-rc
7d0fb76 [Xiangrui Meng] change tree/master to tree/branch-0.9 in docs
ea2b205 [Xiangrui Meng] update version in SparkBuild
162af66 [Xiangrui Meng] Merge remote-tracking branch 'apache/branch-0.9' into v0.9.2-rc
bc87035 [Xiangrui Meng] bump version numbers to 0.9.2
update CHANGES.txt