Enable reference buffer by default.

Change-Id: I21eaa680b0b340da5401a2b893ad9c847495ab79

configure

diff --git a/configure b/configure
index 5b622d7..fc6ac65 100755
--- a/configure
+++ b/configure
@@ -464,6 +464,9 @@
     # Enable filter_7bit by default
     soft_enable filter_7bit
 
+    # Enable reference_buffer by default
+    soft_enable reference_buffer
+
     # Enable daala_ec by default
     ! enabled ans && soft_enable daala_ec