Fix tf.version.GIT_VERSION to return str (instead of bytes)

This fix fixes the issue raised in 24578 where
`tf.version.GIT_VERSION` returns byte instead of string:
```
Python 3.6.7 (default, Oct 22 2018, 11:32:17)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import tensorflow as tf
>>> print(tf.version.GIT_VERSION)
b'v1.12.0-5133-gc343196842'
>>> print(tf.version.COMPILER_VERSION)
4.8.5
>>> print(tf.version.VERSION)
1.13.0-dev20181226
```

This fix fixes 24578.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>