WIP
Test load/save GraphML. Assumes additions in gremlin-node not yet in NPM.
Install gremlin-node package from git to use experimental promises branch.

I'd like to be able to push this gremlin-repl branch to github
and have Travis be able to build it. Since it depends on experimental
features in gremlin-node, we have to be able configure the package
dependencies so that Travis can find the experimental branch.