Forgotten hardcoded path to `app`

This fixes error when Gruntfile was moved to root directory and yeoman.app changed accordingly:

```
Running "compass:dist" (compass) task
Errno::ENOENT on line ["33"] of /usr/local/Cellar/ruby193/1.9.3-p484/lib/ruby/gems/1.9.1/gems/compass-0.12.4/lib/compass/exec/global_options_parser.rb: No such file or directory - /Users/floatdrop/swarming/app
Run with --trace to see the full backtrace

Running "compass:server" (compass) task
Errno::ENOENT on line ["33"] of /usr/local/Cellar/ruby193/1.9.3-p484/lib/ruby/gems/1.9.1/gems/compass-0.12.4/lib/compass/exec/global_options_parser.rb: No such file or directory - /Users/floatdrop/swarming/app
Run with --trace to see the full backtrace
```