Update list of required gems in README.md

I've updated the list of gems that the README says to install, based on my experience trying to install KriKri into Heiðrún and another application.


Of course, now it occurs to me that _if_ turbolinks is really necessary (is a prerequisite of Blacklight, for instance) then wither the krikri or blacklight gemspec file should include it, shouldn't it?  It should be a dependency that's pulled in when you install krikri (or blacklight).