Update package.json.
Remove dependencies on async library.

After the conversion to using supporting Q promises, there was
only one location, in a unit test, where the code still depended
on async. This commit removes that dependency, and removes
async from the package dependencies.
Require node >=0.10.0.

FYI, I tried node 0.8.x on Travis and the unit tests failed. I doubt that anyone needs gremlin-node to run with node 0.8 so I am removing support for it.
Updated README.md for v0.4.0 release.
Merge pull request #9 from jimlloyd/feature/version-0.4.0

Version 0.4.0 -- Promises & various enhancements