Bump tensorflow from 1.12.2 to 1.15.2

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.12.2 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@v1.12.2...v1.15.2)

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

Author: dependabot[bot]

Pipfile

@@ -10,7 +10,7 @@ pylint   = "*"
 v = {editable = true,version = "*"}
 
 [packages]
-tensorflow = "==1.12.2"
+tensorflow = "==1.15.2"
 nni = "==0.9.1.1"
 configargparse = "==0.14.0"
 flask = "*"