message: add TravisCI badge
diff: # RustyHermit - A Rust-based, lightweight unikernel
  [![Build Status](https://git.rwth-aachen.de/acs/public/hermitcore/libhermit-rs/badges/master/pipeline.svg)](https://git.rwth-aachen.de/acs/public/hermitcore/libhermit-rs/pipelines)
  +[![Build Status](https://travis-ci.org/hermitcore/libhermit-rs.svg?branch=master)](https://travis-ci.org/hermitcore/libhermit-rs)
  [![Slack Status](https://radiant-ridge-95061.herokuapp.com/badge.svg)](https://radiant-ridge-95061.herokuapp.com)
  [HermitCore]( http://www.hermitcore.org ) is a [unikernel](http://unikernel.org) targeting a scalable and predictable runtime for high-performance and cloud computing.
  