Fix GSMLocationTime, XBee, A6

Signed-off-by: Sara Damiano <sdamiano@stroudcenter.org>

tools/test_build/test_build.ino

@@ -91,6 +91,8 @@ void loop() {
   client.stop();
 
 #if defined(TINY_GSM_MODEM_HAS_SSL)
+  // modem.addCertificate();  // not yet impemented
+  // modem.deleteCertificate();  // not yet impemented
   TinyGsmClientSecure client_secure(modem);
   TinyGsmClientSecure client_secure2(modem);
   TinyGsmClientSecure client_secure3(modem, 1);