Avoid nil pointer dereference with Geckodriver 0.16+

Geckodriver 0.15 changed the response to NewSession to move all data
under the "Value" field, as per the emerging WebDriver specification.

This issue is only triggered when Geckodriver is used directly.

Fixes #60.

remote.go

@@ -331,7 +331,17 @@ func (wd *remoteWD) NewSession() (string, error) {
 			return "", err
 		}
 
-		wd.id = *reply.SessionID
+		if reply.SessionID != nil {
+			wd.id = *reply.SessionID
+		} else if len(reply.Value) > 0 {
+			value := new(struct {
+				SessionID string
+			})
+			if err := json.Unmarshal(reply.Value, value); err != nil {
+				return "", fmt.Errorf("error unmarshalling value: %v", err)
+			}
+			wd.id = value.SessionID
+		}
 
 		return wd.id, nil
 	}