Remove support for Rails <4.0 and Ruby <1.9.3

Rails 3.2 has been unsupported since Tolk 1.4.
Don't force developers to use a specific Ruby

...but allow them to pin the version themselves.
Update readme to clarify Rails version support

Fixes #72
Merge pull request #76 from zoombody/rails-support

Clarify Rails version support
Merge pull request #77 from zoombody/ruby-version

Don't force developers to use a specific Ruby
Updating changelog, releasing 1.5.2
Remove preloader 'nodoc' call
Update safe_yaml decency to a more conservative option
Dependency is on rails 4.1 now, update preloader calls
Target both Rails 4.0 and 4.1
Restore older preloader syntax for Rails 4 compat
Fix AR preloaders
Make specs pass on Rails 4.1
Run db:migrate before test on travis
Merge pull request #82 from grk/make-specs-pass

Make specs pass on Rails 4.1
Load safe_yaml without patching YAML.load
Don't set safe_yaml global options

This way the users can decide for themselves, and tolk can pass
the desired options to each SafeYAML.load call.
Extract YAML handling to a Tolk::YAML wrapper
Move ya2yaml checking to Tolk::YAML.dump
Remove dead code that was used with Rails < 4