Fix CI setting to pass the test against Node v0.8 / Use SVG badge

* Currently this project fails to pass the test against Node v0.8 [like this](https://travis-ci.org/alex-seville/blanket/builds/26011367).
The only problem in this error is that npm v1.2 doesn't support the `^` version specifier. It's not the problem of Node v0.8 itself.
So I added [the `befire_install` field](http://docs.travis-ci.com/user/build-configuration/#Build-Lifecycle) to install the latest version of npm, before installing the dependencies of Blanket.js.
* Additionally, I changed the image format of the build status badge from PNG to SVG. SVG badges look beautiful on retina displays.

If this Pull Request is merged, #421 and #422 will pass the test.