Ruby 2.3 Support

[jhawthorn]

We seem to have some spec failures running under 2.3

https://circleci.com/gh/jhawthorn/solidus/444

[mamhoff]

It's the tax rate specs. I replaced them in #536 - ready for Ruby 2.3 ;)

[jhawthorn]

Locally I'm also seeing an issue which would be fixed by rubysherpas/paranoia#292

[jhawthorn]

Fixed by #656 and I've released paranoia 2.1.5