install-node: Upgrade yarn to 1.15.2.

In addition to upgrading dependencies being generally useful, this may
fix situations where yarn fails but returns a success status code in the 
presence of an HTTP proxy.

scripts/lib/install-node

@@ -8,7 +8,7 @@ if [ "$TRAVIS" ] ; then
 fi
 YARN_PACKAGE_JSON="$ZULIP_SRV/zulip-yarn/package.json"
 node_version=8.11.1
-yarn_version=1.7.0
+yarn_version=1.15.2
 
 # This is a fix for the fact that nvm uses $HOME to determine which
 # user account's home directory to ~/.config to.  Ideally, we'd have a