puppet: Create directory for redis PID file.

The Redis configuration, and the systemd file for it, assumes there
will be a pid file written to `/var/run/redis/redis.pid`, but
`/var/run/redis` is not created during installation.

Create `/run/redis`; as `/var/run` is a symlink to `/run` on systemd
systems, this is equivalent to `/var/run/redis`.

puppet/zulip/manifests/profile/redis.pp

@@ -60,9 +60,19 @@ class zulip::profile::redis {
     content => template('zulip/zulip-redis.template.erb'),
   }
 
+  file { '/run/redis':
+    ensure  => 'directory',
+    owner   => 'redis',
+    group   => 'redis',
+    mode    => '0755',
+    require => Package[$redis],
+  }
   service { $redis:
     ensure    => running,
-    subscribe => [File[$zulip_redisconf],
-                  Exec['redis']],
+    require   => File['/run/redis'],
+    subscribe => [
+      File[$zulip_redisconf],
+      Exec['redis'],
+    ],
   }
 }