Bump tensorflow in /solutionbox/ml_workbench/xgboost (#727)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.8.0 to 1.15.2.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.8.0...v1.15.2)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Christopher Wilcox <crwilcox@google.com>

Author: dependabot[bot]

solutionbox/ml_workbench/xgboost/setup.py

@@ -27,7 +27,7 @@
   long_description="""
   """,
   install_requires=[
-    'tensorflow==1.8.0',
+    'tensorflow==1.15.2',
     'protobuf==3.4.0',
     'pillow==3.4.1',  # ML Engine does not have PIL installed
     'xgboost==0.6a2',