Cutting release 2.5.0

[Sijie Guo]

Hi all,

Since all the code changes marked for 2.5.0 are merged to master, I am
cutting branch-2.5 and bumping the version to 2.6.0.

Please use 2.6.0 milestone for feature development, 2.5.1 milestone for bug
fixes for 2.5.0.

Thanks,
Sijie