Use mock mailer in dev

We were trying to send emails in dev, which was attempting to use
localhost:25, resulting in errors in the repl.

dev/user.clj

@@ -6,6 +6,7 @@
     [errors :as errors]
     [system :as system]]
    [clojars.db.migrate :as migrate]
+   [clojars.email :as email]
    [clojars.s3 :as s3]
    [clojure.java.io :as io]
    [clojure.tools.namespace.repl :refer [refresh]]
@@ -29,6 +30,7 @@
     (s3/put-object stats-bucket "all.edn" (ByteArrayInputStream. (.getBytes "{}")))
     (assoc (system/new-system (meta-merge (config/config) dev-env))
            :error-reporter (errors/stdout-reporter)
+           :mailer         (email/mock-mailer)
            :repo-bucket    (s3/mock-s3-client)
            :stats-bucket   stats-bucket)))