Installation of nltk and scikit-learn
I met the same problem today. Now I have solved it. 
Because I have installed numPy manually, and I use the command "pip" to install the else package.
Solve way:

find the old version of numPy.
You can import numPy and print the path of it.
delete the folder.
use pip to install again.
I have been trying to install and use scikit-learn and nltk. However, I get the following error while importing anything: 

Traceback (most recent call last):
    File "", line 1, in 
    File "/usr/local/lib/python2.7/site-packages/sklearn/init.py", line 57, in 
      from .base import clone
    File "/usr/local/lib/python2.7/site-packages/sklearn/base.py", line 11, in 
      from .utils.fixes import signature
    File "/usr/local/lib/python2.7/site-packages/sklearn/utils/init.py", line 10, in 
      from .murmurhash import murmurhash3_32
    File "numpy.pxd", line 155, in init sklearn.utils.murmurhash (sklearn/utils/murmurhash.c:5029)
  ValueError: numpy.dtype has the wrong size, try recompiling 

I did a pip uninstall numpy followed by a pip install numpy and also a pip uninstall scikit-learn and again reinstalled it. But the error persists.