title: Not recognizing new distance_threshold parameter for agglomerative clustering
body: <p>The Scikit package doesn't seem to recognize the distance threshold parameter which I am trying to use instead of the n-clusters parameter because I don't know how many clusters I will end up with, it depends on the data. </p>
  
  <p>I am wondering if this is because the distance_threshold parameter was rolled out relatively recently, but I import scikit at the top of the file so I don't see why this would be a problem. Everything works as long as I don't include this parameter.</p>
  
  <pre><code>import matplotlib.pyplot as plt   
  %matplotlib inline
  from sklearn.cluster import AgglomerativeClustering
  
  cluster = AgglomerativeClustering(affinity='euclidean', linkage='ward', 
  distance_threshold = 400.0)  #the last bit here is the problem
  cluster.fit_predict(Revs) 
  labels = np.array(cluster.labels_).tolist()
  </code></pre>
  
  <p>It should run the clustering, instead I get this error message:</p>
  
  <pre><code>**TypeError**: __init__() got an unexpected keyword argument 'distance_threshold'
  </code></pre>