Use latest for Backblaze unversioned downloads

Backblaze do not offer versioned downloads of their installer so
attempting to upgrade the Cask causes issues with mismatched checksums
if the previous installer is still cached.

This means we lose the ability to detect updates but forcibly upgrading
the Cask (e.g. with `brew cask upgrade backblaze`) should now work as
expected.

Casks/backblaze.rb

@@ -1,10 +1,8 @@
 cask "backblaze" do
-  version "7.0.2.464"
-  sha256 "4956a72674d1901a6ec5a98ec0089ebfde743c5bb63e7644c3103e113e26d6d8"
+  version :latest
+  sha256 :no_check
 
   url "https://secure.backblaze.com/mac/install_backblaze.dmg"
-  appcast "https://secure.backblaze.com/api/clientversion.xml",
-          must_contain: "mac_version=\"#{version}\""
   name "Backblaze"
   homepage "https://backblaze.com/"