Make passing Travis badge bright green

This is the other half of #20. I don't have Illustrator, so I just edited the SVG file manually and genereated the PNG using this website: http://www.fileformat.info/convert/image/svg2png.htm

When I tried using the `svg2png` command-line utility, it produced this: ![travis_passing](https://f.cloud.github.com/assets/10308/199947/5a832cdc-80af-11e2-8367-35bd7d3de90a.png)

/cc @laserlemon