Upgrade exim (4.91 -> 4.92)

Note: https://www.us-cert.gov/ncas/current-activity/2019/06/13/Exim-Releases-Security-Patches

That said, I don't believe we've been affected.
Not in a bad way at least, because:
- we run exim as non-root and capabilities dropped
- we run exim in a private Docker network with known trusted relayers
(Synapse and mxisd)

roles/matrix-mailer/defaults/main.yml

@@ -2,7 +2,7 @@ matrix_mailer_enabled: true
 
 matrix_mailer_base_path: "{{ matrix_base_data_path }}/mailer"
 
-matrix_mailer_docker_image: "devture/exim-relay:4.91-r3-0"
+matrix_mailer_docker_image: "devture/exim-relay:4.92-r0-0"
 matrix_mailer_docker_image_force_pull: "{{ matrix_mailer_docker_image.endswith(':latest') }}"
 
 # The user/group that the container runs with.