Fix CI setting to pass the test against Node v0.8 / Add CI test against Node v0.10

* I updated `.travis.yml` to fix [this build error on Travis CI](https://travis-ci.org/iamchrismiller/grunt-casper/builds/27186819). We can simply solve it by updating [npm](https://www.npmjs.org/package/npm) to the latest version. Actually, [this Pull Request passed the test](https://travis-ci.org/iamchrismiller/grunt-casper/builds/27552962).
* I added `v0.10` to the versions of Travis CI tests against. v0.10 is current stable version of Node.
* I updated [duration](https://www.npmjs.org/package/duration), [grunt-contrib-jshint](https://www.npmjs.org/package/grunt-contrib-jshint) and [grunt-contrib-nodeunit](https://www.npmjs.org/package/grunt-contrib-nodeunit).
* I updated image format of the badges in README from SVG to PNG. SVG badges look beautiful on retina displays.