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/setup/install-node

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+set -e
+
+ZULIP_PATH=$(dirname "$0")
+export NVM_DIR=/usr/local/nvm
+if ! [ -e "$NVM_DIR/nvm.sh" ]; then
+    wget -qO- https://raw.githubusercontent.com/creationix/nvm/v0.32.0/install.sh | bash
+fi
+
+source "$NVM_DIR/nvm.sh"
+node_version=6.6.0
+nvm install "$node_version" && nvm alias default "$node_version"
+
+# Fix messed-up uid=500 and group write bits produced by nvm
+n=$(which node)
+n=${n%/bin/node}
+chown -R root:root "$n"
+chmod -R go-w "$n"
+
+# Install node and npm wrappers to /usr/local/bin
+cp "$ZULIP_PATH/../../scripts/setup/node-wrapper" /usr/local/bin/node
+cp "$ZULIP_PATH/../../scripts/setup/npm-wrapper" /usr/local/bin/npm