MB-100: Make GeoCouch safe for future rebar versions

In newer rebar versions, you need to name the "src" directory
explicitly in `src_dirs`, else those files won't get compiled.

Change-Id: Iff510a22e6ade20220ceac3b59e4bf0ef2c7a87f
Reviewed-on: http://review.couchbase.org/18641
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>

rebar.config

@@ -1,4 +1,4 @@
 {erl_opts, [
     % Make sure test/gc_test_util.erl gets compiled
-    {src_dirs, ["test"]}
+    {src_dirs, ["src", "test"]}
 ]}.