fix(diff): diff with nested dashboard element (grafana#41)

As parseDashboards got refactored out, the nesting from the API didn't
line up anymore with the rendered dashboards. This generated a broken
diff.

Ref for the original change: d58bc1a

pkg/grizzly/grafana.go

@@ -189,11 +189,16 @@ func getDashboard(config Config, uid string) (*Board, error) {
 		return nil, err
 	}
 
-	var board Board
-	if err := json.Unmarshal(data, &board); err != nil {
+	type nestedBoard struct {
+		Dashboard Board `json:"dashboard"`
+	}
+	var b nestedBoard
+	if err := json.Unmarshal(data, &b); err != nil {
 		return nil, APIErr{err, data}
 	}
 
+	board := Board{Dashboard: b.Dashboard.Dashboard}
+
 	return &board, nil
 }