Updates frbr? to protect against false frbr groups

I'm not sure what frbrtype = 6 signifies… but it not a valid frbr group
so this updates the detection to exclude type 6.

There was a note about this in a discussion here:
http://www.exlibrisgroup.org/display/PrimoOI/Brief+Search
Merge pull request #9 from JPrevost/update_frbr_detection

Updates frbr? to protect against false frbr groups