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]

docs/install-generic-unix-dev.md

@@ -293,13 +293,10 @@ if [ $(uname) = "OpenBSD" ]; then sudo cp ./puppet/zulip/files/postgresql/zulip_
 ./tools/setup/postgres-init-test-db
 ./tools/do-destroy-rebuild-test-database
 ./manage.py compilemessages
+sudo ./tools/setup/install-node
 npm install
 ```
 
-If `npm install` fails, the issue may be that you need a newer version
-of `npm`.  You can use `npm install -g npm` to update your version of
-`npm` and try again.
-
 To start the development server:
 
 ```