Add :leex and :yecc to mix.exs

Fix warnings when compiling `.xrl` and `.yrl` files.

```
warning: in order to compile .xrl files, you must add "compilers: [:leex] ++ Mix.compilers()" to the "def project" section of couchdbtest's mix.exs
warning: in order to compile .yrl files, you must add "compilers: [:yecc] ++ Mix.compilers()" to the "def project" section of couchdbtest's mix.exs
```

Author: jiahuili430

mix.exs

@@ -54,7 +54,7 @@ defmodule CouchDBTest.Mixfile do
       lockfile: Path.expand("mix.lock", __DIR__),
       deps_path: Path.expand("src", __DIR__),
       build_path: Path.expand("_build", __DIR__),
-      compilers: [:elixir, :app],
+      compilers: [:elixir, :app, :leex, :yecc],
       start_permanent: Mix.env() == :prod,
       build_embedded: Mix.env() == :prod,
       deps: deps(),