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]

tools/test-js-with-node

@@ -3,11 +3,10 @@ set -e
 
 cd "$(dirname "$0")"/..
 
-export NODE_PATH=/usr/lib/nodejs:static
+export NODE_PATH=static
 export PATH=$PATH:node_modules/.bin
 
 INDEX_JS=frontend_tests/zjsunit/index.js
-NODEJS=$(which nodejs || which node)
 ret=0
 
 if [ "$1" = "cover" ]; then
@@ -26,7 +25,7 @@ elif [ "$1" = "-h" -o "$1" = "--help" ]; then
 else
   # Normal testing, no coverage analysis.
   # Run the index.js test runner, which runs all the other tests.
-  "$NODEJS" --stack-trace-limit=100 "$INDEX_JS" $@ || ret=1;
+  node --stack-trace-limit=100 "$INDEX_JS" $@ || ret=1;
 fi
 
 if [ $ret = '0' ]; then