Suppress boost registration warnings in pycaffe (Based on #3960)

[seanbell]

This fixes warnings of the form:

/home/ubuntu/caffe/python/caffe/pycaffe.py:13: RuntimeWarning: to-Python converter for boost::shared_ptr<caffe::Net<float> > already registered; second conversion method ignored.
  from ._caffe import Net, SGDSolver, NesterovSolver, AdaGradSolver, \
/home/ubuntu/caffe/python/caffe/pycaffe.py:13: RuntimeWarning: to-Python converter for boost::shared_ptr<caffe::Blob<float> > already registered; second conversion method ignored.
  from ._caffe import Net, SGDSolver, NesterovSolver, AdaGradSolver, \
/home/ubuntu/caffe/python/caffe/pycaffe.py:13: RuntimeWarning: to-Python converter for boost::shared_ptr<caffe::Solver<float> > already registered; second conversion method ignored.
  from ._caffe import Net, SGDSolver, NesterovSolver, AdaGradSolver, \

It is based on the fix proposed in #3960.

See #3944, #3866, #3575.

I have tested it on Ubuntu 14.04 with Python 2.7.6 and Boost version 1.54.0.1ubuntu1

[shelhamer]

@seanbell that was a quick follow-up—thanks for handling this!

Tested on

• OS X 10.11.4 w/ Python 2.7.11 and Boost 1.60.0
• Ubuntu 15.04 w/ Python 2.7.10 and Boost 1.55
• Ubuntu 16.04 w/ Python 2.7.11 and Boost 1.58

and will merge once CI passes.

[shelhamer]

Re-rolling for Travis not to crash...

[shelhamer]

Thanks @seanbell!