If you see the error shown below on OS X, then there was an issue installing the cffi
package.
c/_cffi_backend.c:15:10: fatal error: 'ffi.h' file not found
#include <ffi.h>
To fix this, try installing libffi via brew:
$ brew install libffi
If you see the error shown below on OS X, then there was an issue installing the cryptography
package.
build/temp.macosx-10.10-x86_64-2.7/_openssl.c:423:10: fatal error: 'openssl/e_os2.h' file not found
#include <openssl/e_os2.h>
To fix this issue, try upgrading Xcode Command Line Tools:
$ xcode-select --install
Then, make sure that openssl is installed and re-link it:
$ brew install openssl
$ brew link openssl --force