installDeps.sh: do not modify package.json when installing dependencies

installDeps.sh's only side effect should be to actually install dependencies
according to a configuration file, and not to modify it.

Adding --no-save to the npm command line forces npm to have a read only access
to package.json.

Old npm versions seem to support --no-XXX style flags even if not documented
(see npm/npm#14285 (comment))

Tested with npm 3.5.2 and 6.1.0

Fixes #3396

bin/installDeps.sh

@@ -73,7 +73,7 @@ echo "Ensure that all dependencies are up to date...  If this is the first time
   cd node_modules
   [ -e ep_etherpad-lite ] || ln -s ../src ep_etherpad-lite
   cd ep_etherpad-lite
-  npm install --loglevel warn
+  npm install --no-save --loglevel warn
 ) || {
   rm -rf node_modules
   exit 1