buildbot: fetch binaryen from mirror

It was just added to the list of mirrors.
diff --git a/buildbot/build.py b/buildbot/build.py
index adcf65a..068fcce 100755
--- a/buildbot/build.py
+++ b/buildbot/build.py
@@ -361,11 +361,8 @@
                         git_repo=SEXPR_GIT)
   SyncOCaml()
   GitCloneFetchCheckout(name='spec', work_dir=SPEC_SRC_DIR, git_repo=SPEC_GIT)
-  # TODO(jfb) binaryen isn't mirrored yet.
-  github_hack = 'https://github.com/WebAssembly/binaryen.git'
   GitCloneFetchCheckout(name='binaryen', work_dir=BINARYEN_SRC_DIR,
-                        git_repo=github_hack)
-  #                      git_repo=BINARYEN_GIT)
+                        git_repo=BINARYEN_GIT)
 
 
 def BuildLLVM():