bug 948644 - add filename and version column to missing_symbols

r? @selenamarie - technically we don't need these to replace modulelist, but anticipate other reports based on this data (see bug for details)
This ALTER change needs to be added to the migration.. in prod I see: 

breakpad=# \d missing_symbols
  Table "public.missing_symbols"
     Column     | Type | Modifiers 
----------------+------+-----------
 date_processed | date | not null
 debug_file     | text | 
 debug_id       | text | 
Number of child tables: 3 (Use \d+ to list them.)