Remove deprecations from gemrc

Removed deprecated bulk_threshold-instance method:
http://www.rubydoc.info/github/rubygems/rubygems/Gem/ConfigFile#bulk_threshold-instance_method

--no-ri --no-rdoc is deprecated, use --no-document
Reference: http://guides.rubygems.org/command-reference/
Merge pull request #546 from jeffwidman/patch-2

Remove deprecations from gemrc