Investigate bentoml.keras deprecation

[rmarquis]

Using bentoml 1.3.15 with tensorflow 1.17.1 (which has a keras interface), I see a deprecation warning:

BentoMLDeprecationWarning: `bentoml.keras` is deprecated since v1.4 and will be removed in a future version.
  bento_model = bentoml.keras.get("celestial_bodies_classifier_model")

I am not sure where that deprecation warning is generated (bentoML is still in 1.3.x), nor if it stlll relevant following the upstream patch (see #194).

Investigate.