Change callback generation to use symbols instead of strings

[brandoncordell]

In Rails 5.1.0.beta1 passing a string to create a callback was deprecated. It's moved past deprecation and not throws an ArgumentError.

I've changed the callback generation to use symbols instead of strings.

See: https://github.com/rails/rails/blob/master/activesupport/lib/active_support/callbacks.rb#L283

[dmitry-mukhin]

@vizvamitra can you look at this please?

[brandoncordell]

I'm not sure if uploadcare has a style guide, it may be cleaner to before_save "check_#{attribute}_for_uuid".to_sym

[vizvamitra]

@dmitry-mukhin we can merge it because this change won't break anything. Also we should bump patch version.

Also we should think about rails 5.1 support since it is already in rc1. I've just tried to run some tests against rails 5.1 and 20% of them are failing, but this is probably because of the tests themself.

[dmitry-mukhin]

@vizvamitra what about earlier rails?

[brandoncordell]

Registering a callback using a symbol is valid back to Rails 3.2 (and probably before that).

http://guides.rubyonrails.org/v3.2/active_record_validations_callbacks.html#callback-registration

[brandoncordell]

@vizvamitra There's also an issue with the generator with Rails 5.1.0.rc1. I'll create an issue and a pull request.

Pull request here: #54

[vizvamitra]

@dmitry-mukhin registering callbacks using symbols is supported since rails 2.3 (or maybe even earlier) so there shouldn't be any problems.

@brandoncordell change in schema.rb seems accidental and not related to the issue. Can you please remove it from the commit?

[vizvamitra]

Closing since fixed in #55