Deprecating update_attribute by cherry-picking b081f6b59fb3f15d12043072a...

Hello,

I believe the ActiveRecord::Base method update_attribute was intended to be deprecated in 4.0.  As you can see from the discussion here:

https://groups.google.com/forum/#!topic/rubyonrails-core/BWPUTK7WvYA/discussion
and
https://github.com/rails/rails/commit/b081f6b59fb3f15d12043072ad9b331ffd2bc56e

it was originally deprecated in 3.2 and removed in 4.0.  After that, I believe these changes were reverted so as to not deprecate functionality in patch releases:

https://github.com/rails/rails/commit/81542f95d25825a7d3eff87d6f706661bf553b18

However, I think the intent was still to have it deprecated in 4.0.

This pull request adds that deprecation.  Please let me know if this is indeed desired, and if so if any changes are needed.

Likely interested parties: @fxn and @steveklabnik

Thanks!
Andrew