Get the save path for the excel from an environment variable.

- Engineyard creates a new release directory for each deploy
- If the server isn't restarted, `Rails.root` points to the old directory
- Excel generation fails because we use `Rails.root` to get the directory to save into.

This pull-request allows setting the excel-save-directory using application.yml
We should set the excel save directory as `/data/surveyweb/current/public`