[zathura] zathura-pdf-mupdf: update for the mupdf to 1.9 API

[Leonardo Taccari]

[...]
I have tested them for several days with mupdf-1.9a without noticing any
problems.
[...]

...but of course I've forgotten some part for config.mk! This will just
change `-lmujs' to `-lmupdfthird'.

Please also bump the requirements in README to `mupdf (>= 1.9)' (patch
not provided)!


Please let me know if you need more information and/or different format
of the patches!
Ciao,
L.