Update Arcanist config to point to reviews.llvm.org

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@210558 91177308-0d34-0410-b5e6-96231b3b80d8

.arcconfig

@@ -1,4 +1,4 @@
 {
   "project_id" : "libcxx",
-  "conduit_uri" : "http://llvm-reviews.chandlerc.com/"
+  "conduit_uri" : "http://reviews.llvm.org/"
 }