title: RandomUnderSampler' object has no attribute 'fit_resample'
body: <p>I am using <code>RandomUnderSampler</code> from <code>imblearn</code>, but I get the following error. Any ideas? Thanks</p>
  
  <pre><code>from imblearn.under_sampling import RandomUnderSampler
  print('Initial dataset shape %s' % Counter(y.values.squeeze()))
  rus = RandomUnderSampler(random_state=42)
  X_undersampled, y_undersampled = rus.fit_resample(X, y)
  y_undersampled = y_undersampled.squeeze()
  </code></pre>
  
  <p>output:</p>
  
  <pre><code>Initial dataset shape Counter({0: 2499739, 1: 1558})
  
  ---------------------------------------------------------------------------
  AttributeError                            Traceback (most recent call last)
  &lt;ipython-input-18-4fe9dcfbd68e&gt; in &lt;module&gt;
        1 print('Initial dataset shape %s' % Counter(y.values.squeeze()))
        2 rus = RandomUnderSampler(random_state=42)
  ----&gt; 3 X_undersampled, y_undersampled = rus.fit_resample(X, y)
        4 y_undersampled = y_undersampled.squeeze()
        5 
  
  AttributeError: 'RandomUnderSampler' object has no attribute 'fit_resample'
  </code></pre>
  
  <p>main libraries I am using:</p>
  
  <pre><code>imbalanced-learn==0.3.3
  pandas==0.24.2
  numpy==1.15.4
  scikit-learn==0.19.2
  </code></pre>