installDeps.sh: don't nuke all installed plugins on update

This is a modification of PR #3474 by rohieb <rohieb@rohieb.name>.

When this script is run after an update, and something fails during the npm
run, the installed plugins were deleted, too.

In this version what gets deleted is only the directory that contains the
direct dependencies of the core Etherpad, ie:
  node_modules/ep_etherpad-lite/node_modules

bin/installDeps.sh

@@ -116,7 +116,7 @@ echo "Ensure that all dependencies are up to date...  If this is the first time
   cd ep_etherpad-lite
   npm install --no-save --loglevel warn
 ) || {
-  rm -rf node_modules
+  rm -rf src/node_modules
   exit 1
 }