Update gemfile and README.md

* README.md was only a stub; added some real text.

* Updated Travis-CI link to point to new location https://github.com/zendesk/credit_card_sanitizer

* Updated gemspec e-mail address to `ggrossman@zendesk.com` as `eac@zendesk.com` is no longer valid.

/cc @rtatsumi @vkmita