Minor doc update: protobuf should be installed with Python support on Mac

On Mac OS X, if one installs protobuf as recommended with

    brew install protobuf

then pycaffe import fails:

    [~/s/caffe/python] $ python
    Python 2.7.8 (default, Jul  2 2014, 10:14:58)
    [GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.40)] on darwin
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import sys; caffe_root = '../'; sys.path.insert(0, caffe_root + 'python'); import caffe
    ImportError                               Traceback (most recent call last)
    <ipython-input-3-a65a880449a1> in <module>()
    ----> 1 import sys; caffe_root = '../'; sys.path.insert(0, caffe_root + 'python'); import caffe

    /Users/jason/s/caffe/python/caffe/__init__.py in <module>()
    ----> 1 from .pycaffe import Net, SGDSolver
          2 from .classifier import Classifier
          3 from .detector import Detector
          4 import io

    /Users/jason/s/caffe/python/caffe/pycaffe.py in <module>()
          9
         10 from ._caffe import Net, SGDSolver
    ---> 11 import caffe.io
         12
         13 # We directly update methods from Net here (rather than using composition or

    /Users/jason/s/caffe/python/caffe/io.py in <module>()
          4 from skimage.transform import resize
          5
    ----> 6 from caffe.proto import caffe_pb2
          7
          8

    /Users/jason/s/caffe/python/caffe/proto/caffe_pb2.py in <module>()
          2 # source: caffe/proto/caffe.proto
          3
    ----> 4 from google.protobuf.internal import enum_type_wrapper
          5 from google.protobuf import descriptor as _descriptor
          6 from google.protobuf import message as _message

    ImportError: No module named google.protobuf.internal

The solution is just to use the following brew command instead:

    brew install --with-python protobuf