Fix test

Pathname#write is unsupported in Ruby <= 2.0

Author: jonleighton

lib/spring/test/acceptance_test.rb

@@ -528,9 +528,9 @@ def exec_name
 
       test "server boot timeout" do
         app.env["SPRING_SERVER_COMMAND"] = "sleep 1"
-        app.spring_client_config.write %(
+        File.write("#{app.spring_client_config}", %(
           Spring::Client::Run.const_set(:BOOT_TIMEOUT, 0.1)
-        )
+        ))
 
         assert_failure "bin/rails runner ''", stderr: "timed out"
       end