Remove upper bound on Railsties Dependency to simplify future Rails updates

[kanejamison]

This PR removes the upper bounded limit on the gemspec for the railsties dependency.

The upper bound is not maintained actively on this gem to match the development pace of Rails, which means customers are forced to fork this gem and maintain their own copies in order to stay up to date on their own applications.

While there is a chance that the wysiwyg-rails gem will not be compatible with future updates to railsties - this gem will need to fix those conflicts regardless. The rest of the time, the upper bound just adds friction to Froala customers, and actively prevents us from identifying upcoming gem issues and reporting them back here.

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Here is a related conversation where the Mike Perham, the author and maintainer of Sidekiq, once of the most heavily used free and paid background jobs systems for Rails, explaining why they choose not to declare upper limits in their gemspec requirements (my initial post was about another gem facing the same issue):

"This is exactly why Sidekiq only ever specifies ">=" in its gemspec version constraints. As a library in your application, I'd rather be loose and assume compatibility so I don't force unnecessary upgrades on you."