Deploys: --skip-themes when updating WP template_root

Some WP themes may throw errors if loaded in this context,
e.g., some PHP classes may not be available with `--skip-plugins`.
It is not necessary to load themes when updating WP options for
template_root and stylesheet root, so it is safer to `--skip-themes`.

CHANGELOG.md

@@ -1,4 +1,5 @@
 ### HEAD
+* Deploys: `--skip-themes` when updating WP `template_root` ([#849](https://github.com/roots/trellis/pull/849))
 * Option to install WP-CLI packages ([#837](https://github.com/roots/trellis/pull/837))
 * Update WP-CLI to 1.2.1 ([#838](https://github.com/roots/trellis/pull/838))
 * Auto-install Vagrant plugins ([#829](https://github.com/roots/trellis/pull/829))

roles/deploy/hooks/finalize-after.yml

@@ -12,7 +12,7 @@
     when: project.update_db_on_deploy | default(update_db_on_deploy)
 
   - name: Get WP theme template root
-    command: wp option get template_root --skip-plugins
+    command: wp option get template_root --skip-plugins --skip-themes
     args:
       chdir: "{{ deploy_helper.current_path }}"
     register: wp_template_root