bump rails version 3.2.15 to 3.2.16

Rails 3.2.16 has been released.
http://weblog.rubyonrails.org/2013/12/3/Rails_3_2_16_and_4_0_2_have_been_released/

Bump rails v3.2.15 to v3.2.16.
There was no change in the status of the CI before and after work.

Please merge this.