Cannot get boot-http :not-found handler to work when running `boot dev`

build.boot

(set-env!
 :source-paths    #{"src/cljs" "src/clj"}
 :resource-paths  #{"resources"}
:target-path "target"
 :dependencies '[[org.clojure/clojure "1.7.0"]
                 [adzerk/boot-cljs          "1.7.228-1"   :scope "test"]
                 [com.cemerick/piggieback "0.2.1" :scope "test"]
                 [weasel "0.7.0" :scope "test"]
                 [org.clojure/tools.nrepl "0.2.12" :scope "test"]
                 [adzerk/boot-cljs-repl     "0.3.0"      :scope "test"]
                 [adzerk/boot-reload        "0.4.5"      :scope "test"]
                 [pandeiro/boot-http        "0.7.3"      :scope "test"]
                 [org.clojure/clojurescript "1.7.228"]
                 [crisptrutski/boot-cljs-test "0.2.2-SNAPSHOT" :scope "test"]
                 [reagent "0.6.0-alpha"]
                 [cljs-ajax "0.5.3"]
                 [secretary "1.2.3"]                        ; todo remove
                 [bidi "1.25.1"]
                 [com.andrewmcveigh/cljs-time "0.4.0"]
                 [kibu/pushy "0.3.6"]
                 [cljsjs/moment "2.10.6-1"]
                 [re-frame "0.7.0-alpha"]
                 [adzerk/boot-template "1.0.0"]
                 [matchbox "0.0.8-SNAPSHOT"]])

(require
  '[adzerk.boot-cljs :refer [cljs]]
  '[adzerk.boot-cljs-repl :refer [cljs-repl start-repl]]
  '[adzerk.boot-reload :refer [reload]]
  '[pandeiro.boot-http :refer [serve]]
  '[clojure.java.io :as io]
  '[reps.server :refer [not-found-handler]])

(deftask build []
  (comp (speak)
        (cljs)))

(deftask run []
  (comp (serve :not-found 'not-found-handler) ;; note this
        (watch)
        (cljs-repl)
        (reload)
        (build)))

(deftask production []
  (task-options! cljs {:optimizations :advanced})
  identity)

(deftask development []
  (task-options! cljs {:optimizations :none :source-map true}
                 reload {:on-jsload 'web.app/mount})
  identity)

(deftask prod []
  (comp (production)
        (cljs)))

(deftask dev
  "Simple alias to run application in development mode"
  []
  (comp (development)
        (configure)
        (run)))

server.clj

(ns reps.server
  (:require [ring.util.response :refer [file-response]]))

(defn not-found-handler [request]
  (assoc-in
    (file-response "target/index.html")
    [:headers "Content-Type"]
    "text/html;charset=utf8"))