text: Title: Piping not working on Python 3.5
  Body: I'm trying to clean the test notebooks by using a pipe but it doesn't work on Python 3.5. I just get empty result:
  
  ```
  $ cat tests/test_metadata.ipynb | nbstripout 
  
  ```
  
  Running `nbstripout tests/test_metadata.ipynb` correctly cleans the notebook inplace.
  
  I tried with Python 2.7 and then it outputs the normal cleaned notebook when piping. Any ideas what could be wrong?
  
  ```
  $ pip freeze
  decorator==4.0.11
  docopt==0.6.2
  docutils==0.13.1
  ipython==6.0.0
  ipython-genutils==0.2.0
  jedi==0.10.2
  jsonschema==2.5.1
  jupyter-core==4.3.0
  nbformat==4.3.0
  nbstripout==0.3.0
  path.py==10.1
  pexpect==4.2.1
  pickleshare==0.7.4
  prompt-toolkit==1.0.14
  ptyprocess==0.5
  Pygments==2.2.0
  simplegeneric==0.8.1
  six==1.10.0
  testpath==0.3
  traitlets==4.3.2
  wcwidth==0.1.6
  ```