resource type assigned to gdor items

- assigns format_main_ssim for item records
- assigns format_main_ssim to collection records based on their MODS, in addition to the "Archive/Manuscript" value added because it is a collection record
- updates some of the gems used in this branch, esp. stanford-mods commit
- updated validation methods and tests for format_main_ssim 

@lmcglohon 

See also https://jirasul.stanford.edu/jira/browse/INDEX-128