Bump tensorflow from 1.12.2 to 1.15.4 in /Tools/WinMLDashboard/public

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.12.2 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@v1.12.2...v1.15.4)

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

Author: dependabot[bot]

Tools/WinMLDashboard/public/requirements.txt

@@ -2,7 +2,7 @@ coremltools_windows==2.0b1
 onnxmltools==1.7.0
 onnxconverter-common==1.7.0
 h5py==2.8.0
-tensorflow==1.12.2
+tensorflow==1.15.4
 tf2onnx==1.6.3
 Keras==2.2.4
 Keras-Applications==1.0.6