formula_installer: prevent MaximumMacOSRequirement leakage

Read the discussion in https://github.com/Homebrew/homebrew-core/pull/3703. If you
have a better idea, please file a competing PR. I'm sick to death of discussion.

Closes #662.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/formula_installer.rb

@@ -308,6 +308,7 @@ class FormulaInstaller
 
     req_map.each_pair do |dependent, reqs|
       reqs.each do |req|
+        next if dependent.installed? && req.name == "maximummacos"
         puts "#{dependent}: #{req.message}"
         fatals << req if req.fatal?
       end