message: updated banner image for v3.1
diff: </h5>
  <p align="center">
  <a href="https://github.com/gitbrent/PptxGenJS/">
  - <img alt="PptxGenJS Sample Slides" title="PptxGenJS Sample Slides" src="https://raw.githubusercontent.com/gitbrent/PptxGenJS/gh-pages/img/readme_banner.jpg"/>
  + <img alt="PptxGenJS Sample Slides" title="PptxGenJS Sample Slides" src="https://raw.githubusercontent.com/gitbrent/PptxGenJS/gh-pages/img/readme_banner.png"/>
  </a>
  </p>
  <br/>
  