Bump tensorflow from 1.5 to 1.15.4

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.5 to 1.15.4.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.5.0...v1.15.4)

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

Author: dependabot[bot]

requirements.txt

@@ -1,6 +1,6 @@
 scikit-image
 numpy
-tensorflow==1.13.2
+tensorflow==1.15.4
 nibabel
 pandas
 medpy