remove pg warning from tests

removes the following warning which showed up on macs and travis:

unknown OID 3220: failed to recognize type of 'pg_current_wal_insert_lsn'.
It will be treated as String

converts the value to a string

lib/extensions/ar_adapter/ar_dba.rb

@@ -12,7 +12,7 @@ def spid
   end
 
   def xlog_location
-    select_value("SELECT pg_current_wal_insert_lsn()")
+    select_value("SELECT pg_current_wal_insert_lsn()::varchar")
   end
 
   def xlog_location_diff(lsn1, lsn2)