Update doc-present to use changed variable names of doc-view

Hi,

when I wanted to use doc-present I noticed that it failed with the message that 
doc-view-current-cache-dir and doc-view-current-converter-processes are both void.
A quick investigation showed, that these seem to have a double dash prefix:
doc-view--

I added two checks if a variable with the old name is bound and if it is
void, it uses the new variable names.