Update version numbers for TensorFlow 1.15.1

[tensorflow-jenkins]

Before merging this PR, please double check that it has correctly updated
core/public/version.h, tools/pip_package/setup.py, and
tensorflow/tensorflow.bzl. Also review the execution notes below:

Major: 1 -> 1
Minor: 15 -> 15
Patch: 0 -> 1

WARNING: Below are potentially instances of lingering old version string 
"1.15.0" in source directory "tensorflow/" that are not updated by this script. 
Please check them manually!
tensorflow/tools/pip_package/setup.py:65:1.15.0
tensorflow/tools/pip_package/setup.py:98:1.15.0
tensorflow/tools/ci_build/release/common.sh:242:1.15.0
tensorflow/tools/ci_build/release/common.sh:245:1.15.0
tensorflow/lite/experimental/micro/examples/micro_speech/README.md:591:1.15.0
tensorflow/lite/experimental/micro/examples/micro_speech/train_speech_model.ipyn
b:126:1.15.0

WARNING: Below are potentially instances of lingering old version string 
"1.15.0" in source directory "tensorflow/" that are not updated by this script. 
Please check them manually!
tensorflow/tools/pip_package/setup.py:65:1.15.0
tensorflow/tools/pip_package/setup.py:98:1.15.0
tensorflow/tools/ci_build/release/common.sh:242:1.15.0
tensorflow/tools/ci_build/release/common.sh:245:1.15.0
tensorflow/lite/experimental/micro/examples/micro_speech/README.md:591:1.15.0
tensorflow/lite/experimental/micro/examples/micro_speech/train_speech_model.ipyn
b:126:1.15.0