hardcocded path to /engineyard/bin

while working on new code for the scripts we use `/engineyard/custom` instead of `/engineyard/bin` for the new version of scripts (https://github.com/engineyard/eycap/blob/master/lib/eycap/recipes/unicorn.rb#L50), the simplest solution would be to extract a variable like: `engienyard_bin` which defaults to `/engineyard/bin` but can be set by user to any other location:
```ruby
set :engineyard_bin, "/engineyard/custom"
```