Install py4j. (#1099)

The latest version of `hyperopt` (0.2.6) requires `py4j` but doesn't list it
as a requirement.

This caused our tests to start failing.

`py4j` is a package to access Java objects.

http://b/206631257

Author: rosbo

Dockerfile.tmpl

@@ -239,6 +239,8 @@ RUN pip install mpld3 && \
     pip install haversine && \
     pip install toolz cytoolz && \
     pip install plotly && \
+    # b/206631257: hyperopt requires py4j >= 0.2.6 requires py4j but doesn't list it as a dependency. Remove once hyperopt properly list it.
+    pip install py4j && \
     pip install hyperopt && \
     pip install fitter && \
     pip install langid && \