Release future resources properly

This prevented all future objects from being properly garbage collected.
diff --git a/c_src/main.c b/c_src/main.c
index 2060b72..8321261 100644
--- a/c_src/main.c
+++ b/c_src/main.c
@@ -119,6 +119,7 @@
     }
 
     ret = enif_make_resource(env, f);
+    enif_release_resource(f);
     return T3(env, ATOM_erlfdb_future, ref, ret);
 }