Fix np.load call for np v1.16.3 (keras-team#12714)

Numpy 1.16.3 changed the default value for the allow_pickle flag.

So this load call breaks.

https://nvd.nist.gov/vuln/detail/CVE-2019-6446

https://github.com/numpy/numpy/blob/v1.16.2/numpy/lib/npyio.py#L288
https://github.com/numpy/numpy/blob/v1.16.3/numpy/lib/npyio.py#L292

np.__version__ = 1.16.3
keras.version = 2.2.4
keras.datasets.imdb.load_data()          

```
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-2-154d5969a0d5> in <module>
----> 1 k.datasets.imdb.load_data()

~/venv3/lib/python3.6/site-packages/keras/datasets/imdb.py in load_data(path, num_words, skip_top, maxlen, seed, start_char, oov_char, index_from, **kwargs)
     57                     file_hash='599dadb1135973df5b59232a0e9a887c')
     58     with np.load(path) as f:
---> 59         x_train, labels_train = f['x_train'], f['y_train']
     60         x_test, labels_test = f['x_test'], f['y_test']
     61 

~/venv3/lib/python3.6/site-packages/numpy/lib/npyio.py in __getitem__(self, key)
    260                 return format.read_array(bytes,
    261                                          allow_pickle=self.allow_pickle,
--> 262                                          pickle_kwargs=self.pickle_kwargs)
    263             else:
    264                 return self.zip.read(key)

~/venv3/lib/python3.6/site-packages/numpy/lib/format.py in read_array(fp, allow_pickle, pickle_kwargs)
    690         # The array contained Python objects. We need to unpickle the data.
    691         if not allow_pickle:
--> 692             raise ValueError("Object arrays cannot be loaded when "
    693                              "allow_pickle=False")
    694         if pickle_kwargs is None:

ValueError: Object arrays cannot be loaded when allow_pickle=False
```

keras/datasets/imdb.py

@@ -55,7 +55,7 @@ def load_data(path='imdb.npz', num_words=None, skip_top=0,
     path = get_file(path,
                     origin='https://s3.amazonaws.com/text-datasets/imdb.npz',
                     file_hash='599dadb1135973df5b59232a0e9a887c')
-    with np.load(path) as f:
+    with np.load(path, allow_pickle=True) as f:
         x_train, labels_train = f['x_train'], f['y_train']
         x_test, labels_test = f['x_test'], f['y_test']