Ensure swig is installed

[ianvmfarms]

running build_ext

building 'M2Crypto.__m2crypto' extension

swigging SWIG/_m2crypto.i to SWIG/_m2crypto_wrap.c

swig -python -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -I/usr/include -I/usr/include/openssl -includeall -modern -o SWIG/_m2crypto_wrap.c SWIG/_m2crypto.i

unable to execute swig: No such file or directory

error: command 'swig' failed with exit status 1

----------------------------------------
Cleaning up...
Command /Users/ian/Envs/exepyvenv/bin/python -c "import setuptools, tokenize;__file__='/Users/ian/Envs/exepyvenv/build/M2Crypto/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /var/folders/zq/wm6wrrp54lg9knxyzrks0y3c0000gn/T/pip-xiEZeU-record/install-record.txt --single-version-externally-managed --compile --install-headers /Users/ian/Envs/exepyvenv/include/site/python2.7 failed with error code 1 in /Users/ian/Envs/exepyvenv/build/M2Crypto
Storing debug log for failure in /Users/ian/.pip/pip.log

On OS X I accomplished this with "brew update ; brew install swig".