Include aarch64-linux in platform list (#31)

Author: arjun810

lib/difftastic/upstream.rb

@@ -7,6 +7,7 @@ module Upstream
 		NATIVE_PLATFORMS = {
 			"arm64-darwin" => "difft-aarch64-apple-darwin.tar.gz",
 			"arm64-linux" => "difft-aarch64-unknown-linux-gnu.tar.gz",
+			"aarch64-linux" => "difft-aarch64-unknown-linux-gnu.tar.gz",
 			"x86_64-darwin" => "difft-x86_64-apple-darwin.tar.gz",
 			"x86_64-linux" => "difft-x86_64-unknown-linux-gnu.tar.gz",
 		}.freeze