Add compatibility with Rails 5+

[Nowaker]

There's nothing in prototype-rails code that prevents it from working in Rails 5+. Remove the restriction from the gemspec so any legacy applications willing to use new Rails 5 features have an upgrade path without having to rewrite their code.

Before it gets merged (if ever gets merged), anyone who needs to use this version of prototype-rails is welcome to use my fork:

gem 'prototype-rails', git: 'https://github.com/Nowaker/prototype-rails.git', branch: '5.0'