Use different Nginx versions when deploying to different Heroku stacks (Cedar or Cedar-14)

In Cedar-14, the version of Nginx included here fails. This is because Cedar-14 uses Ubuntu 14.04 which comes with libssl1.0.0 whereas the Nginx binary previously included here had been compiled with libssl0.9.8.

After testing for a while I found that the following shared libraries were missing in order for Nginx to work in Ubuntu 14.04:
- libssl1.0.0
- libcrypto1.0.0
- libc6 (version glibc_2.14)

The Nginx binary version included for Cedar is the previous one 1.5.11.1, which I've left unchanged.
The version included for Cedar-14 is the latest stable of Openresty as of today: 1.7.4.1