When installing puma on OSX 10.11 El Capitan, if you are facing issues compiling mini_ssl.c

fatal error: 'openssl/bio.h' file not found.

all you need to do is to specify

--with-opt-dir

You can make this applicable for all your Gemfiles:

bundle config build.puma --with-opt-dir=/usr/local/opt/openssl

Thanks to Jeremy for this