Move graylog to dedicated repo

The new repo uses gopkg.in for versioning.
This is the continuation of logrus moving hooks to dedicated repos.

The old repo
https://github.com/gemnasium/logrus-hooks/tree/master/graylog
is deprecated, as we don't want to share things there too (import
airbrake while I just want graylog). It will be removed in a few
months.

README.md

@@ -210,7 +210,7 @@ Note: Syslog hook also support connecting to local syslog (Ex. "/dev/log" or "/v
 | [Logrusly](https://github.com/sebest/logrusly) | Send logs to [Loggly](https://www.loggly.com/) |
 | [Slackrus](https://github.com/johntdyer/slackrus) | Hook for Slack chat. |
 | [Journalhook](https://github.com/wercker/journalhook) | Hook for logging to `systemd-journald` |
-| [Graylog](https://github.com/gemnasium/logrus-hooks/tree/master/graylog) | Hook for logging to [Graylog](http:https://graylog2.org/) |
+| [Graylog](https://github.com/gemnasium/logrus-graylog-hook) | Hook for logging to [Graylog](http:https://graylog2.org/) |
 | [Raygun](https://github.com/squirkle/logrus-raygun-hook) | Hook for logging to [Raygun.io](http:https://raygun.io/) |
 | [LFShook](https://github.com/rifflock/lfshook) | Hook for logging to the local filesystem |
 | [Honeybadger](https://github.com/agonzalezro/logrus_honeybadger) | Hook for sending exceptions to Honeybadger |