Configuring with class method instead of constant

I was getting an error when configuring the gem with an initializer in Rails, since constants cannot be changed once they are declared. Instead, we should configure with a class variable.

This is potentially a breaking change, since `MailgunEmailValidator::MAILGUN_PUBLIC_KEY = 'key'` will no longer work to configure the gem. Instead, you need to do `MailgunEmailValidator.mailgun_public_key = 'key'`.

Thanks!