[change_migrations_to_use_change_method] Change method :up/:down to use :change.

As described in https://github.com/bbatsov/rails-style-guide#migrations.
Merge pull request #195 from paulopatto/change_migrations_to_use_change_method

[change_migrations_to_use_change_method] Change method ActiveRecord::Migration :up/:down to use :change.