text: Title: [utils.py] IndexError: indices are out-of-bounds
  Body: Hi,
  
  I probably did something wrong but... I really don't find it.
  
  During the fit call, I have this exception:
  
  ```
  X (301, 2) Y (301, 2)
  ---------------------------------
  Run id: JTHQIT
  Log directory: /tmp/tflearn_logs/
  ---------------------------------
  Training samples: 301
  Validation samples: 0
  --
  Exception in thread Thread-3:
  Traceback (most recent call last):
    File "/home/shazz/projects/anaconda/envs/tensorflow/lib/python3.5/threading.py", line 914, in _bootstrap_inner
      self.run()
    File "/home/shazz/projects/anaconda/envs/tensorflow/lib/python3.5/threading.py", line 862, in run
      self._target(*self._args, **self._kwargs)
    File "/home/shazz/projects/anaconda/envs/tensorflow/lib/python3.5/site-packages/tflearn/data_flow.py", line 186, in fill_feed_dict_queue
      data = self.retrieve_data(batch_ids)
    File "/home/shazz/projects/anaconda/envs/tensorflow/lib/python3.5/site-packages/tflearn/data_flow.py", line 221, in retrieve_data
      utils.slice_array(self.feed_dict[key], batch_ids)
    File "/home/shazz/projects/anaconda/envs/tensorflow/lib/python3.5/site-packages/tflearn/utils.py", line 187, in slice_array
      return X[start]
    File "/home/shazz/projects/anaconda/envs/tensorflow/lib/python3.5/site-packages/pandas/core/frame.py", line 2051, in __getitem__
      return self._getitem_array(key)
    File "/home/shazz/projects/anaconda/envs/tensorflow/lib/python3.5/site-packages/pandas/core/frame.py", line 2096, in _getitem_array
      return self.take(indexer, axis=1, convert=True)
    File "/home/shazz/projects/anaconda/envs/tensorflow/lib/python3.5/site-packages/pandas/core/generic.py", line 1669, in take
      convert=True, verify=True)
    File "/home/shazz/projects/anaconda/envs/tensorflow/lib/python3.5/site-packages/pandas/core/internals.py", line 3932, in take
      indexer = maybe_convert_indices(indexer, n)
    File "/home/shazz/projects/anaconda/envs/tensorflow/lib/python3.5/site-packages/pandas/core/indexing.py", line 1872, in maybe_convert_indices
      raise IndexError("indices are out-of-bounds")
  IndexError: indices are out-of-bounds
  ```
  
  I really started from the titanic example, I just took a different dataset (weight, height => sex), that I clean using pandas, that's the only difference.
  
  Code:
  
  ```
  import tflearn
  import data_importer
  
  X, Y = data_importer.load_data(0)    
  print("X", X.shape, "Y", Y.shape)
  
  # Build neural network
  net = tflearn.input_data(shape=[None, 2])
  net = tflearn.fully_connected(net, 32)
  net = tflearn.fully_connected(net, 32)
  net = tflearn.fully_connected(net, 2, activation='softmax')
  net = tflearn.regression(net)
  
  # Define model
  model = tflearn.DNN(net)
  
  # Start training (apply gradient descent algorithm)
  model.fit(X, Y, n_epoch=10, batch_size=1, show_metric=True)
  ```
  
  Data importer code and data are available here:
  https://github.com/shazz/tflearn-experiments/tree/master/cdc
  
  Any help welcome...... is a a bug in my code (probably) or in tflearn ? 
  Thanks !
  