replace install_xtuple.sh with install-dev

Absolutely right about `4_8_x`. I'd been intending to replace that with `$TRAVIS_BRANCH` as soon as this was stable. 

The explicit path to npm is necessary for reasons I don't understand, to be honest. I'd prefer if it weren't in there, but it failed strangely when done the intuitive way for reasons that I imagine are somehow TravisCI-related.