Bump tensorflow from 1.13.1 to 1.15.0

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.13.1 to 1.15.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.13.1...v1.15.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -8,4 +8,4 @@ tqdm==4.28.1
 matplotlib==2.2.3
 pyaudio==0.2.11
 keras==2.2.4
-tensorflow==1.13.1
+tensorflow==1.15.0