Print "Running via Spring preloader" message to stderr

Not stdout; stdout is for the output of the actual command being run,
stderr is for extra messages/warnings like this.

Author: jonleighton

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 * Fix problems with the implementation of the new "Running via Spring preloader"
   message (see #456, #457)
+* Print "Running via Spring preloader" message to stderr, not stdout
 
 ## 1.6.1
 

lib/spring/application.rb

@@ -157,7 +157,7 @@ def serve(client)
         IGNORE_SIGNALS.each { |sig| trap(sig, "DEFAULT") }
         trap("TERM", "DEFAULT")
 
-        puts "Running via Spring preloader in process #{Process.pid}" unless Spring.quiet
+        STDERR.puts "Running via Spring preloader in process #{Process.pid}" unless Spring.quiet
 
         ARGV.replace(args)
         $0 = command.exec_name

lib/spring/test/acceptance_test.rb

@@ -103,14 +103,14 @@ def without_gem(name)
       end
 
       test "tells the user that spring is being used when used automatically via binstubs" do
-        assert_success "bin/rails runner ''", stdout: "Running via Spring preloader in process"
-        assert_success app.spring_test_command, stdout: "Running via Spring preloader in process"
+        assert_success "bin/rails runner ''", stderr: "Running via Spring preloader in process"
+        assert_success app.spring_test_command, stderr: "Running via Spring preloader in process"
       end
 
       test "does not tell the user that spring is being used when used automatically via binstubs but quiet is enabled" do
         File.write("#{app.user_home}/.spring.rb", "Spring.quiet = true")
         assert_success "bin/rails runner ''"
-        refute_output_includes "bin/rails runner ''", stdout: 'Running via Spring preloader in process'
+        refute_output_includes "bin/rails runner ''", stderr: 'Running via Spring preloader in process'
       end
 
       test "test changes are picked up" do