text: Title: Precision-Recall Curve does not show class labels
  Body: **Describe the bug**
  
  When labels are passed into the `PrecisionRecallCurve` the visualization is not drawn correctly.
  
  **To Reproduce**
  
  ```python
  from yellowbrick.classifier import PrecisionRecallCurve 
  from yellowbrick.dataset import load_game 
  
  from sklearn.preprocessing import LabelEncoder 
  from sklearn.naive_bayes import MultinomialNB 
  from sklearn.model_selection import train_test_split as tts 
  
  # Load the dataset and label encode the target 
  data = load_game() 
  X = data.iloc[:, data.columns != 'outcome']
  y = LabelEncoder().fit_transform(data['outcome'])
  
  # Create train test splits 
  X_train, X_test, y_train, y_test = tts(X, y, test_size=0.2, shuffle=True)
  
  oz = PrecisionRecallCurve(
      MultinomialNB(), per_class=True, iso_f1_curves=True, fill_area=False, 
      micro=False, classes=["loss", "draw", "win"]
  )
  oz.fit(X_train, y_train)
  oz.score(X_test, y_test)
  oz.poof()
  ```
  
  **Dataset**
  
  I used the game multi-label dataset from the UCI Machine Learning repository, as wrangled by the yellowbrick datasets module.
  
  **Expected behavior**
  
  When the target, `y` is label encoded (e.g. via the `LabelEncoder`) to classes 0, 1, and 2 and class names are passed in via the `classes` param of the visualizer, the legend should display the class names. However, in this case the visualization does not appear at all:
  
  ![figure_1](https://user-images.githubusercontent.com/745966/45109335-3c59a800-b10d-11e8-9386-e74de86282eb.png)
  
  
  **Traceback**
  
  No exception is raised, however, the following warning is issued:
  
  ```
  /Users/benjamin/.pyenv/versions/3.6.2/envs/yellowbrick3/lib/python3.6/site-packages/numpy/lib/arraysetops.py:522: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
    mask |= (ar1 == a)
  /Users/benjamin/.pyenv/versions/3.6.2/envs/yellowbrick3/lib/python3.6/site-packages/sklearn/metrics/ranking.py:444: RuntimeWarning: invalid value encountered in true_divide
    recall = tps / tps[-1]
  objc[46460]: Class FIFinderSyncExtensionHost is implemented in both /System/Library/PrivateFrameworks/FinderKit.framework/Versions/A/FinderKit (0x7fffb473cc90) and /System/Library/PrivateFrameworks/FileProvider.framework/OverrideBundles/FinderSyncCollaborationFileProviderOverride.bundle/Contents/MacOS/FinderSyncCollaborationFileProviderOverride (0x12814ecd8). One of the two will be used. Which one is undefined.
  ```
  
  **Desktop (please complete the following information):**
   - OS: macOS High Sierra 10.13.6
   - Python Version 3.6.2
   - Yellowbrick Version 0.9 develop 
  
  **Additional context**
  
  This was a known bug during development but was left in as a rare case. 
  