Fix test

For some reason, the previous code didn't work on the CI using Ruby
2.3.0 (it worked on every other Ruby version, and locally).

When I debugged it, I found that the shell wasn't finding the "spring"
executable. (I have no idea why, but it's presumably
environment-specific.)

Author: jonleighton

lib/spring/test/acceptance_test.rb

@@ -516,7 +516,7 @@ def exec_name
       test "booting a foreground server" do
         FileUtils.cd(app.root) do
           assert !spring_env.server_running?
-          app.run "spring server &"
+          assert_success "bin/spring server &"
 
           Timeout.timeout(10) do
             sleep 0.1 until spring_env.server_running? && spring_env.socket_path.exist?