Fix rare YAML parsing problems in Dimension config

It's been reported that YAML parsing errors
would occur on certain Ansible/Python combinations for some reason.

It appears that a bare `{{ matrix_dimension_admins }}` would sometimes
yield things like `[u'@user:domain.com', ..]` (note the `u` string prefix).

To prevent such problems, we now explicitly serialize with `|to_json`.

roles/matrix-dimension/defaults/main.yml

@@ -61,7 +61,7 @@ matrix_dimension_configuration_yaml: |
 
   # These users can modify the integrations this Dimension supports.
   # To access the admin interface, open Dimension in Riot and click the settings icon.
-  admins: {{ matrix_dimension_admins }}
+  admins: {{ matrix_dimension_admins|to_json }}
 
   # IPs and CIDR ranges listed here will be blocked from being widgets.
   # Note: Widgets may still be embedded with restricted content, although not through Dimension directly.