bump node version in travis yml

I'm not sure that's true. The CI right now runs `install_xtuple.sh` anyway, and not the `xtuple-server`, to set up. (That's something I'm fairly keen to normalize.)

We could possibly leave this field empty and it might pick up the right node version, but right now this line of code is coercing the build environment to operate on a version of node that we don't want.