mitmproxy build failures on OS X 10.10

By default, it fails building Pillow because it's picking up an Xlib dependency via Tk but /opt/X11/include is apparently not in the include path:

https://gist.github.com/anonymous/6ee660d0cb79de2976f8

Adding `depends_on :x11` to mitmproxy.rb allows Pillow to build but it will fail on the cryptography module, which depends on libffi:

https://gist.github.com/b6763e148f04a51fcb69