Stop decoding Auth0 client secrets by default

[drewcimino]

This pull request fixes #149.

As of December 2016, Auth0 is not Base64-encoding the client secrets. Existing secrets on Auth0 are remaining that way, but newly generated ones won't be.

I propose changing the config/knock.rb template to not use JWT.base64url_decode, to maintain out-of-the-box functionality with new Auth0 implementations. That line is specifically written for Auth0 implementations and anyone with a Base64-encoded secret is already using it, so changing the default shouldn't affect them.