Revert "Revert "syslog-ng: Update from 3.5.6 to 3.6.1""

This reverts commit 8e1072fd60f9dcf5eac0bb2d604258f121b658d8.
Up syslog-ng-incubator to work with syslog-ng 3.6

Also disable riemann extension, since it is now part of 3.6.
Merge pull request #4863 from rickynils/syslog-ng-3.6

syslog-ng 3.6
add html2text and html2text python script