title: Dask not installing graphviz dependency
body: <p>I'm getting an error when I try to import <code>dask.dot</code> that it can't find the graphviz install. However, both graphviz and pygraphviz are installed. </p>
  
  <pre><code>balter@exalab3:~$ conda install dask
  Fetching package metadata ...........
  Solving package specifications: ..........
  
  # All requested packages already installed.
  # packages in environment at /home/.../miniconda3:
  #
  dask                      0.13.0                   py35_0    conda-forge
  balter@exalab3:~$ python
  Python 3.5.3 | packaged by conda-forge | (default, Jan 23 2017, 19:01:48)
  [GCC 4.8.2 20140120 (Red Hat 4.8.2-15)] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  &gt;&gt;&gt; import dask.dot
  Traceback (most recent call last):
    File "/home/.../miniconda3/lib/python3.5/site-packages/dask/utils.py", line 63, in import_required
      return import_module(mod_name)
    File "/home/.../miniconda3/lib/python3.5/importlib/__init__.py", line 126, in import_module
      return _bootstrap._gcd_import(name[level:], package, level)
    File "&lt;frozen importlib._bootstrap&gt;", line 986, in _gcd_import
    File "&lt;frozen importlib._bootstrap&gt;", line 969, in _find_and_load
    File "&lt;frozen importlib._bootstrap&gt;", line 956, in _find_and_load_unlocked
  ImportError: No module named 'graphviz'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/home/.../miniconda3/lib/python3.5/site-packages/dask/dot.py", line 11, in &lt;module&gt;
      graphviz = import_required("graphviz", "Drawing dask graphs requires the "
    File "/home/.../miniconda3/lib/python3.5/site-packages/dask/utils.py", line 65, in import_required
      raise RuntimeError(error_msg)
  RuntimeError: Drawing dask graphs requires the `graphviz` python library and the `graphviz` system library to be installed.
  &gt;&gt;&gt;
  balter@exalab3:~$ conda install graphviz
  Fetching package metadata ...........
  Solving package specifications: ..........
  
  # All requested packages already installed.
  # packages in environment at /home/.../miniconda3:
  #
  graphviz                  2.38.0                        4    bioconda
  </code></pre>
  
  <p>How do I resolve this problem?</p>