figtree: add livecheck

By default, livecheck checks the Git tags for `figtree` but this is
currently returning an unstable version as newest (1.4.5pre). The
related release is marked as pre-release on GitHub
(https://github.com/rambaut/figtree/releases/tag/v1.4.5pre).

This PR addresses the issue by adding a `livecheck` block that uses
the `GithubLatest` strategy, which correctly returns 1.4.4 as the
newest version. The cask uses a GitHub release asset, so checking
releases (instead of Git tags) is the correct approach anyway.

figtree: add desc

Casks/f/figtree.rb

@@ -4,8 +4,14 @@ cask "figtree" do
 
   url "https://github.com/rambaut/figtree/releases/download/v#{version}/FigTree.v#{version}.dmg"
   name "FigTree"
+  desc "Phylogenetic tree viewer"
   homepage "https://github.com/rambaut/figtree/"
 
+  livecheck do
+    url :url
+    strategy :github_latest
+  end
+
   app "FigTree v#{version}.app"
   qlplugin "QuickLook Plugin/FigTreeQuickLookPlugin.qlgenerator"
 end