Merge pull request #3243 from reitermarkus/PATH

Use `PATH` where possible.

Library/Homebrew/dev-cmd/update-test.rb

@@ -81,7 +81,7 @@ module Homebrew
       safe_system "git", "reset", "--hard", start_commit
 
       # update ENV["PATH"]
-      ENV["PATH"] = "#{curdir}/bin:#{ENV["PATH"]}"
+      ENV["PATH"] = PATH.new(ENV["PATH"]).prepend(curdir/"bin")
 
       # run brew update
       oh1 "Running brew update..."

Library/Homebrew/utils.rb

@@ -264,7 +264,7 @@ end
 
 def with_system_path
   old_path = ENV["PATH"]
-  ENV["PATH"] = "/usr/bin:/bin"
+  ENV["PATH"] = PATH.new("/usr/bin", "/bin")
   yield
 ensure
   ENV["PATH"] = old_path