Don't show server_tokens

Showing server tokens reveals the nginx version number to would-be attackers. The version number is an easy way to figure out security exploits.
Merge pull request #31 from a-warner/turn_off_server_tokens

Don't show server_tokens
Make target file name explicit
Include `STACK` environment variable in file name
Vendor binary for "cedar-14" stack
Merge pull request #35 from tt/support-cedar-14

Support "cedar-14"
Merge branch 'master' of https://github.com/ryandotsmith/nginx-buildpack