Chdir to the application directory before starting

This fixed an issue where unicorn 4.7.0 blew up because it couldn't find
the `config.ru` file on Ubuntu 12.04 LTS.

System info:
- ruby 2.0.0p353 (2013-11-22 revision 43784) [x86_64-linux]
- unicorn v4.7.0
- 3.8.0-35-generic #50~precise1-Ubuntu SMP Wed Dec 4 17:25:51 UTC 2013 x86_64