message: Added Travis build status badge to README.md
diff: # TensorFlow I/O
  +[![Travis-CI Build Status](https://travis-ci.org/tensorflow/io.svg?branch=master)](https://travis-ci.org/tensorflow/io)
  +
  TensorFlow I/O is a collection of file systems and file formats that are not
  available in TensorFlow's built-in support.
  