Avoid method name collision

https://github.com/blase/redactor-rails/commit/2864a72c07d82c4066e58e4cffdbba2663e0184f
Command: rails generate redactor:install throws an error.
Since Rails 4.0.4 there is a method called create_migration in Rails::Generators::Migration. Changing the name solves this problem.
Use SCSS import vs sprockets require for compatibility with bourbon and compass