boost version hack fix.

[seewalker]

As seen in this caffe issue, the construction of caffe.Net objects can fail with certain versions of boost and string encodings. I ran into the error message:

ArgumentError: Python argument types in
    Net.__init__(Net, unicode, unicode, int)
did not match C++ signature:
    __init__(boost::python::api::object, std::string, std::string, int) 

This error occurs when using the kaixhin/caffe docker image of caffe. The explicit call to str which this pull request adds gets this style transfer project working out of the box in this environment. I think this is significant because this is a docker image recommended in the caffe installation docs.

[fzliu]

Thanks!