Bump tensorflow from 1.9.0 to 1.15.2 in /pertbio

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.9.0 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@v1.9.0...v1.15.2)

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

Author: dependabot[bot]

pertbio/setup.py

@@ -17,7 +17,7 @@
     url="https://github.com/dfci/CellBox",
     packages=['pertbio'],
     python_requires='>=3.6',
-    install_requires=['tensorflow==1.15.0', 'numpy==1.16.0', 'pandas==0.24.2'],
+    install_requires=['tensorflow==1.15.2', 'numpy==1.16.0', 'pandas==0.24.2'],
     tests_require=['pytest', 'pandas'],
     setup_requires=['pytest-runner', "pytest"],
     zip_safe=True,