Use semver to sort changelogs

If you have two changelog files named, for example, `v0.9.3.md` and `v0.10.0.md`, they will be [sorted incorrectly](https://github.com/aponxi/grunt-readme-generator/blob/master/tasks/readme_generator.coffee#L120), resulting in a changelog in your `README.md` that is not the most recent.

I have changed this behavior to sort instead using the [semver](https://www.npmjs.org/package/semver) module, to ensure the latest version is always correct.

The diff is a bit of a mess because of all the test output, but the meat of the change is [in `tasks/readme_generator.coffee`](tasks/readme_generator.coffee)