Commented in 8078f73c.
Index renames are no longer needed in Rails 4.0:
> In Rails 4.0 when a column or a table is renamed the related indexes
> are also renamed. If you have migrations which rename the indexes,
> they are no longer needed.
See: http://guides.rubyonrails.org/upgrading_ruby_on_rails.html
[ci skip]
Signed-off-by: Alex Coles <alex@alexbcoles.com>