fix: avoid create_migration method conflict

rails4 run `rails g simple_captcha` throws error:
```
/usr/local/var/rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/bundler/gems/simple-captcha-e0fc6ae2d155/lib/generators/simple_captcha_generator.rb:18:in `create_migration': wrong number of arguments (3 for 0) (ArgumentError)
	from /usr/local/var/rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/railties-4.1.5/lib/rails/generators/migration.rb:63:in `migration_template'
```
because of method conflict
see https://github.com/pludoni/simple-captcha/commit/f0677ed6c11c9fdabe58031a7249b3645e4ba221