Merge pull request #4449 from MikeMcQuaid/bintray-fallback

global: handle HOMEBREW_BOTTLE_DOMAIN fallback.

Library/Homebrew/global.rb

@@ -25,8 +25,12 @@ HOMEBREW_USER_AGENT_CURL = ENV["HOMEBREW_USER_AGENT_CURL"]
 HOMEBREW_USER_AGENT_RUBY = "#{ENV["HOMEBREW_USER_AGENT"]} ruby/#{RUBY_VERSION}-p#{RUBY_PATCHLEVEL}".freeze
 HOMEBREW_USER_AGENT_FAKE_SAFARI = "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_3) AppleWebKit/602.4.8 (KHTML, like Gecko) Version/10.0.3 Safari/602.4.8".freeze
 
-HOMEBREW_BOTTLE_DEFAULT_DOMAIN = ENV["HOMEBREW_BOTTLE_DEFAULT_DOMAIN"]
-HOMEBREW_BOTTLE_DOMAIN = ENV["HOMEBREW_BOTTLE_DOMAIN"]
+# Bintray fallback is here for people auto-updating from a version where
+# HOMEBREW_BOTTLE_DEFAULT_DOMAIN isn't set.
+HOMEBREW_BOTTLE_DEFAULT_DOMAIN = ENV["HOMEBREW_BOTTLE_DEFAULT_DOMAIN"] ||
+                                 "https://homebrew.bintray.com"
+HOMEBREW_BOTTLE_DOMAIN = ENV["HOMEBREW_BOTTLE_DOMAIN"] ||
+                         HOMEBREW_BOTTLE_DEFAULT_DOMAIN
 
 require "extend/fileutils"