upgrade zoltar to 0.5.6

[mattnworb]

This re-applies #2254 now that zoltar is using Tensorflow 1.15 (spotify/zoltar#193)

[codecov]

Codecov Report

Merging #2318 into master will decrease coverage by 1.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2318      +/-   ##
==========================================
- Coverage   70.06%   68.73%   -1.33%     
==========================================
  Files         240      238       -2     
  Lines        7767     7747      -20     
  Branches      365      398      +33     
==========================================
- Hits         5442     5325     -117     
- Misses       2325     2422      +97

Impacted Files	Coverage Δ
...scala/com/spotify/scio/bigquery/client/Cache.scala	0% <0%> (-66.67%)	⬇️
.../spotify/scio/bigquery/BigQueryPartitionUtil.scala	0% <0%> (-52.5%)	⬇️
.../spotify/scio/bigquery/client/BigQueryConfig.scala	0% <0%> (-41.18%)	⬇️
.../scala/com/spotify/scio/bigquery/StorageUtil.scala	0% <0%> (-31.71%)	⬇️
...com/spotify/scio/bigquery/types/BigQueryType.scala	65% <0%> (-30%)	⬇️
...otify/scio/bigquery/syntax/ScioContextSyntax.scala	33.33% <0%> (-27.28%)	⬇️
...com/spotify/scio/bigquery/types/TypeProvider.scala	52.17% <0%> (-21.74%)	⬇️
...n/scala/com/spotify/scio/bigquery/BigQueryIO.scala	37.86% <0%> (-3.4%)	⬇️
...la/com/spotify/scio/bigquery/client/BigQuery.scala	28.94% <0%> (-2.64%)	⬇️
...la/com/spotify/scio/bigquery/client/TableOps.scala	0% <0%> (-1.77%)	⬇️
... and 4 more

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[regadas]

Closes #2319