travis: remove node 0.6 testing

I'm not saying, that we want to drop support for Node v0.6, but I would like to drop *active* support for it, in terms of backwards compatibility issues, if any.

The last version of the 0.6 branch is `v0.6.8` and it was published on 20 Jan 2012. That's 2 years ago. People should've really got their shit together and update to a newer version.

It seems like even the npm registry is moving away from 0.6 as no `npm install` will work without additional help. That is why all Travis builds for 0.6 are failing. And that is, why I want to remove the test runner for 0.6. It just costs Travis additional ressources, that could be used otherwise.