Adjust version string for node 0.8.x

Hi! I'm having trouble building the project on node 0.8.x because of the way the version string is written in `package.json`. 

It is confusing that:
- `engines.node` advertises that aws-sdk runs on node >= 0.6.0
- `.travis.yml` does not test on any node version other than 0.10.x
- some of the `devDependencies` are explicitly >= 0.10.x (e.g. [eslint](https://github.com/eslint/eslint/blob/master/package.json#L77))

cc @yhahn