Update to the latest version of kramdown

Note that this required some slight changes to the tests due to subtle differences in kramdown's behaviour:

* There is a bug in kramdown that means &amp; is not correctly converted to it's numeric equivilant.
* three full stops now get converted to a &hellip; in newer versions of kramdown so the test strings needed updating to better reflect real usage.

A bug report has been filed with kramdown on the conversion of certain entities to their numeric equivalent. This is not directly a problem for our usage as we do not use `:numeric` output, only `:symbolic`.

This will resolve a number of bugs in govspeak rendering, including this one related to rendering multiple references to footnotes: https://www.agileplannerapp.com/boards/173808/cards/5428