Bump version to v1.9.0 for plugin peerDependencies

This allows plugins to depend on the not-yet-released API by bumping
their `peerDependencies` to `>=1.9.0`.

IMPORTANT: v1.9.0 IS NOT RELEASED YET. I tried to bump the version to
1.9.0-alpha.0 instead, but unfortunately that doesn't satisfy
`>=1.8.6` which would break just about every plugin.

src/package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "ep_etherpad-lite",
-  "version": "1.8.16",
+  "version": "1.9.0",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {

src/package.json

@@ -248,6 +248,6 @@
     "test": "mocha --timeout 120000 --recursive tests/backend/specs ../node_modules/ep_*/static/tests/backend/specs",
     "test-container": "mocha --timeout 5000 tests/container/specs/api"
   },
-  "version": "1.8.16",
+  "version": "1.9.0",
   "license": "Apache-2.0"
 }