Fix `bundle exec unicorn -c config/unicorn.rb: No such file or directory`

As reported by and @ilove8 @fabiokung, launching a Ruby app now shows the following error:

```
bin/start-nginx: line 36: bundle exec unicorn -c config/unicorn.rb: No such file or directory
```

To fix this, @fabiokung [suggested](https://github.com/ryandotsmith/nginx-buildpack/pull/6/files#r4828674) removing the quotes around `"$COMMAND"` on line 36 of `bin/start-nginx`, which solves the problem

For reference, this is my Procfile:

```
web: bin/start-nginx bundle exec unicorn -c config/unicorn.rb
```