title: ipywidgets interact does not work
body: <p>I am trying <code>ipywidgets.interact</code> <a href="http://ipywidgets.readthedocs.io/en/latest/examples/Using%20Interact.html" rel="nofollow noreferrer">hello world example</a> but the expected element does not show up.</p>
  
  <p>This is the code:</p>
  
  <pre><code> from __future__ import print_function
   from ipywidgets import interact, interactive, fixed, interact_manual
   import ipywidgets as widgets
  
   def f(x):
      return x
  
   interact(f, x= 'test')
  </code></pre>
  
  <p>These are the session parameters:</p>
  
  <pre><code>CPython 2.7.14
  IPython 5.6.0
  
  ipywidgets 7.2.0
  
  compiler   : GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)
  system     : Darwin
  release    : 15.6.0
  machine    : x86_64
  processor  : i386
  CPU cores  : 4
  interpreter: 64bit
  </code></pre>