Update Doxie.app to latest

The previous link to 2.6 no longer works as it’s now http://cdn.getdoxie.com/resources/files/Doxie_2_6_1523.dmg — but I think it’s more robust to go through their PHP redirect script.
Merge pull request #5673 from sorbits/Doxie_latest

Update Doxie.app to latest