Update yajl-ruby

I'm getting segmentation faults on Travis on both Ruby 1.9.3 and 2.1.2,
updating yajl-ruby to 1.2.1 seems to fix the issue
Update changelog
Merge branch 'master' into update_yajl-ruby
Merge pull request #22 from bmarini/update_yajl-ruby

Update yajl-ruby