title: predict_proba does not output probability
body: <p>I am following </p>
  
  <p><a href="https://blog.keras.io/building-powerful-image-classification-models-using-very-little-data.html" rel="nofollow noreferrer">https://blog.keras.io/building-powerful-image-classification-models-using-very-little-data.html</a></p>
  
  <p>specificly: <a href="https://gist.github.com/ritazh/a7c88875053c1106e407300fc4f1d8d6" rel="nofollow noreferrer">https://gist.github.com/ritazh/a7c88875053c1106e407300fc4f1d8d6</a></p>
  
  <p>When I try and get the actual P values for the prediction using model.predict_proba(x) I get [[1.]] or [[0.]], not a decimal value like 0.69.
  Is this because I am using a sigmod activation with only two classes or something wrong with my setup?</p>
  
  <p>Thanks in advance.</p>