This command fails for me on Yosemite. I have Xcode.
It says Failed building wheel for cffi
Command "/Users/me/.local/share/letsencrypt/bin/python2.7 -c "import setuptools........ failed with error code 1 in /private/var/folders/5r/77hy8f_14y1_c9vkzkj5k2i40000gn/T/pip-build-6UUdnc/cffi