provision: Use NVM to install node and npm.

NVM takes a specific node version and installs the node package and
a corresponding compatible npm package.

We use it in a somewhat hackish way to install node/npm globally with
a pinned version, since that's how we actually want to consume node in
our development environment.

Other details:
- Travis CI now is configured to use the version of node installed by
provision; the easiest way to do this was to sabotage the existing node
installation.
- jsdom is upgraded to a current version, which both requires recent
node and also is required for the tests to pass with recent node.
This fixes running the node tests on Xenial.

Fixes #1498.

[tweaked by tabbott]

scripts/setup/node-wrapper

@@ -0,0 +1,4 @@
+#!/bin/bash
+export NVM_DIR="/usr/local/nvm"
+[ -e "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh"
+node "$@"

scripts/setup/npm-wrapper

@@ -0,0 +1,4 @@
+#!/bin/bash
+export NVM_DIR="/usr/local/nvm"
+[ -e "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh"
+npm "$@"