Investigate warnings from Travis build

[ageorgou]

We have at least the following:

1. Deprecation warning for NumPy (note that numpy is installed by default on Travis, not sure which version)

   [NbConvertApp] Converting notebook ch08performance/040cython.ipynb to notebook
   [NbConvertApp] Executing notebook with kernel: python3
   In file included from /home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/numpy/core/include/numpy/ndarraytypes.h:1809:0,
                from /home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/numpy/core/include/numpy/ndarrayobject.h:18,
                from /home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/numpy/core/include/numpy/arrayobject.h:4,
                from /home/travis/.cache/ipython/cython/_cython_magic_47a24c072aa661d40ade61b5d2f07dad.c:587:
   /home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/numpy/core/include/numpy/npy_1_7_deprecated_api.h:15:2: warning: #warning "Using deprecated NumPy API, disable it by " "#defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION" [-Wcpp]

2. Deprecation in nbformat invocation

   python nbmerge.py [list of all the notebooks]
   /home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/IPython/nbformat.py:13: ShimWarning: The `IPython.nbformat` package has been deprecated since IPython 4.0. You should import from nbformat instead.
   "You should import from nbformat instead.", ShimWarning)

3. Validation warnings (I think for every notebook)

   [NbConvertApp] Converting notebook ch00python/00pythons.nbconvert.ipynb to notebook
   /home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/nbformat/validator.py:251: UserWarning: No schema for validating v2 notebooks

None seem too worrisome or tricky, but it would be good to investigate (or ignore in the case of the third?)