CLC-3685 Roll back to working version of nokogiri

https://jira.ets.berkeley.edu/jira/browse/CLC-3685

After updating nokogiri to 1.6.2.1 the deploy to calcentral-dev failed. There were these errs in the torquebox.log, which I missed somehow when I ran it on my localhost:

(LoadError) load error: nokogiri/nokogiri -- java.lang.NoClassDefFoundError: com/sun/org/apache/xpath/internal/VariableStack

I spent a while googling, but found nothing. nokogiri's installation documentation didn't help either. I'm going to roll back to 1.5.9 (which also requires downgrading our omniauth-cas gem) for the time being.