Switch over to using github.com/go-jira/jira, from gopkg.in

There should be no reason to use gopkg.in versioned imports now that
we're using go modules. I think, IANAE.

gopkg.in kind of gets in the way of modules, as it only pulls over
tagged releases from github.com -- this then means that you need to use
go modules 'replace' syntax in the go.mod to use a non-versioned commit
or branch. This is feasible, but kind of ugly.

go modules defaults to pulling the latest version, so the default
behavior is the same as when pulling go-jira.v1 from gopkg.in.

session.go

@@ -4,7 +4,7 @@ import (
 	"bytes"
 	"encoding/json"
 
-	"gopkg.in/Netflix-Skunkworks/go-jira.v1/jiradata"
+	"github.com/go-jira/jira/jiradata"
 )
 
 type AuthProvider interface {