elixir: use a requirement with a default formula for erlang

Formula/elixir.rb

@@ -1,5 +1,29 @@
 require 'formula'
 
+class ErlangInstalled < Requirement
+  fatal true
+  default_formula 'erlang'
+  env :userpaths
+
+  satisfy {
+    erl = which('erl') and begin
+      `#{erl} -noshell -eval 'io:fwrite("~s~n", [erlang:system_info(otp_release)]).' -s erlang halt | grep -q '^R1[6789]'`
+      $?.exitstatus == 0
+    end
+  }
+
+  def message; <<-EOS.undent
+    Erlang R16 is required to install.
+
+    You can install this with:
+      brew install erlang
+
+    Or you can use an official installer from:
+      http://www.erlang.org/
+    EOS
+  end
+end
+
 class Elixir < Formula
   homepage 'http://elixir-lang.org/'
   url  'https://github.com/elixir-lang/elixir/archive/v0.10.2.tar.gz'
@@ -7,7 +31,7 @@ class Elixir < Formula
 
   head 'https://github.com/elixir-lang/elixir.git'
 
-  depends_on 'erlang'
+  depends_on ErlangInstalled
 
   def install
     system "make"