fix module naming in go.mod (#68)

The module name needs to start with a url, usually of the
service that hosts the repo. Otherwise it can't be fetched without
adding replace directives in users' go.mod files, for example:

go get: github.com/bshuster-repo/[email protected] updating to
github.com/bshuster-repo/[email protected]: parsing go.mod:
module declares its path as: bshuster-repo/logrus-logstash-hook
       but was required as: github.com/bshuster-repo/logrus-logstash-hook

go.mod

@@ -1,4 +1,4 @@
-module bshuster-repo/logrus-logstash-hook
+module github.com/bshuster-repo/logrus-logstash-hook
 
 go 1.16