Revert "[FLINK-6122] Add Travis build status to README"

This reverts commit 486f7249.

Removed after the "[DISCUSS] TravisCI status on GitHub Page" discussion on
the mailing list.

This closes #3577.

README.md

 # Apache Flink
-[![Build Status](https://api.travis-ci.org/apache/flink.svg)](https://api.travis-ci.org/apache/flink.svg)
-
 
 Apache Flink is an open source stream processing framework with powerful stream- and batch-processing capabilities.