Tweeper.php: fix Invalid Character Error when converting Instagram json to XML

Converting Instagram json data to XML was failing with the following
message:

  PHP Fatal error:  Uncaught DOMException: Invalid Character Error in
  /usr/share/php/Symfony/Component/Serializer/Encoder/XmlEncoder.php:445

This was caused by some item starting with a number which resulted in
invalid XML element names.

Remove the items containing the problematic names from the json data
before converting to XML.

Also stop handling the "knobs" element which does not seem to be there
anymore.