Roll AWS SDK version back to 1.15.0

Hello @giannic, 

Please review the following commits I made in branch 'xiao-rollback-sdk'.

27a244dcdf5f2dcd603cc686db3b771ec659ff13 (2014-03-02 23:19:45 -0800)
Roll AWS SDK version back to 1.15.0
We experienced performance regression in the 1.18.0 version of AWS SDK,
so we have to roll back to 1.15.0, which doesn't have the regression
and supports GSI.

R=@giannic