Update GCS connector from 1.6.1 to 1.6.3.

[janlugt]

This changes the GCS batch request endpoint to the new per-api endpoint
(https://github.com/GoogleCloudPlatform/bigdata-interop/blob/branch-1.6.x/gcs/CHANGES.txt),
as the old endpoint is being deprecated
(https://developers.googleblog.com/2018/03/discontinuing-support-for-json-rpc-and.html).

[codecov-io]

Codecov Report

Merging #4590 into master will decrease coverage by 0.266%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             master     #4590       +/-   ##
==============================================
- Coverage     79.84%   79.574%   -0.266%     
+ Complexity    16958     16911       -47     
==============================================
  Files          1062      1062               
  Lines         61677     61677               
  Branches       9983      9983               
==============================================
- Hits          49243     49079      -164     
- Misses         8539      8708      +169     
+ Partials       3895      3890        -5

Impacted Files	Coverage Δ	Complexity Δ
...s/spark/ParallelCopyGCSDirectoryIntoHDFSSpark.java	0% <0%> (-75.51%)	0% <0%> (-17%)
...nder/tools/spark/pipelines/PrintVariantsSpark.java	0% <0%> (-66.667%)	0% <0%> (-2%)
...oadinstitute/hellbender/utils/test/XorWrapper.java	13.043% <0%> (-65.217%)	2% <0%> (-7%)
...oadinstitute/hellbender/utils/gcs/BucketUtils.java	54.194% <0%> (-25.806%)	30% <0%> (-9%)
...nder/tools/spark/BaseRecalibratorSparkSharded.java	0% <0%> (-22.807%)	0% <0%> (-2%)
...titute/hellbender/utils/test/MiniClusterUtils.java	78.947% <0%> (-10.526%)	6% <0%> (-1%)
...adinstitute/hellbender/engine/ReadsDataSource.java	89.394% <0%> (-3.03%)	61% <0%> (-2%)
...broadinstitute/hellbender/utils/test/BaseTest.java	61.94% <0%> (-2.985%)	30% <0%> (-3%)
...der/engine/spark/datasources/ReadsSparkSource.java	79.487% <0%> (-2.564%)	44% <0%> (ø)
...te/hellbender/engine/spark/VariantWalkerSpark.java	72.34% <0%> (-2.128%)	14% <0%> (ø)
... and 2 more

[droazen]

👍 thanks for the PR!