title: Error: Command '['dot', '-V']' returned non-zero exit status -5 When using Iris
body: <p>After I installed the package <code>Iris</code> with <code>conda install -c scitools iris</code>,</p>
  
  <p>Whether using Ipython console or Jupyter notebook, the <code>iris</code> was always failed to import. The error subroutine shows like:</p>
  
  <pre><code> ---------------------------------------------------------------------------
  CalledProcessError                        Traceback (most recent call last)
  &lt;ipython-input-1-005053b25f37&gt; in &lt;module&gt;()
  ----&gt; 1 import iris
  
  /Users/TEST/anaconda/lib/python2.7/site-packages/iris/__init__.pyc in &lt;module&gt;()
      110 import iris.cube
      111 import iris._constraints
  --&gt; 112 import iris.fileformats
      113 import iris.io
      114 
  
  /Users/TEST/anaconda/lib/python2.7/site-packages/iris/fileformats/__init__.py in &lt;module&gt;()
       23 from six.moves import (filter, input, map, range, zip)  # noqa
       24 
  ---&gt; 25 from iris.io.format_picker import (FileExtension, FormatAgent,
       26                                    FormatSpecification, MagicNumber,
       27                                    UriProtocol, LeadingLine)
  
  /Users/TEST/anaconda/lib/python2.7/site-packages/iris/io/__init__.py in &lt;module&gt;()
       31 
       32 import iris.fileformats
  ---&gt; 33 import iris.fileformats.dot
       34 import iris.cube
       35 import iris.exceptions
  
  /Users/TEST/anaconda/lib/python2.7/site-packages/iris/fileformats/dot.py in &lt;module&gt;()
       41             # Check PATH
       42             subprocess.check_output([_DOT_EXECUTABLE_PATH, '-V'],
  ---&gt; 43                                     stderr=subprocess.STDOUT)
       44         except OSError:
       45             _DOT_EXECUTABLE_PATH = None
  
  /Users/TEST/anaconda/lib/python2.7/subprocess.pyc in check_output(*popenargs, **kwargs)
      571         if cmd is None:
      572             cmd = popenargs[0]
  --&gt; 573         raise CalledProcessError(retcode, cmd, output=output)
      574     return output
      575 
  
  CalledProcessError: Command '['dot', '-V']' returned non-zero exit status -5
  </code></pre>