Fix np.float -> np.floating change

While running core_rnn_cell_test:
```
bazel test -s --verbose_failures --config=opt //tensorflow/contrib/rnn:core_rnn_cell_test
```
Noticed the following warning:
```
FutureWarning: Conversion of the second argument of issubdtype from `float` to `np.floating` is deprecated. In future, it will be treated as `np.float64 == np.dtype(float).type`.
```

This fix fixes the above warning.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>