minor changes to the acceptance tests

I tried to get the acceptance tests running. On my way I made a couple of very small changes to assist in the debugging process. Now the acceptance tests should run directly after a git clone + bundle. (assuming you use `rake` to trigger everything).

While I got the tests running I didn't get them to pass. The first time I run them (shut down all running processes) I get:

```
  1) Failure:
test_app_gets_reloaded_when_preloaded_files_change(AppTest) [/Users/senny/Projects/spring/test/acceptance/app_test.rb:169]:
expected 'RuntimeError: omg' to be printed to stdout. But it wasn't, the stdout is:


  2) Failure:
test_app_recovers_when_a_boot-level_error_is_introduced(AppTest) [/Users/senny/Projects/spring/test/acceptance/app_test.rb:188]:
The run should not be successful but it was

  3) Failure:
test_code_changes_in_pre-referenced_app_files_are_picked_up(AppTest) [/Users/senny/Projects/spring/test/acceptance/app_test.rb:144]:
1.104887 was not less than 0.5 of 1.115265

8 tests, 18 assertions, 3 failures, 0 errors, 0 skips
```

Once the processes run I get the following:

```
  1) Failure:
test_app_recovers_when_a_boot-level_error_is_introduced(AppTest) [/Users/senny/Projects/spring/test/acceptance/app_test.rb:188]:
The run should not be successful but it was

  2) Failure:
test_code_changes_in_pre-referenced_app_files_are_picked_up(AppTest) [/Users/senny/Projects/spring/test/acceptance/app_test.rb:144]:
1.080713 was not less than 0.5 of 1.00083

8 tests, 20 assertions, 2 failures, 0 errors, 0 skips
```

I wonder if we should kill all the processes before a test run, to make sure we get a clean state. I will continue to debug the problems and If I find more, I'll submit another PR.