Add instructions for importing data from Heroku

Since removing Heroku from the Gemfile (and therefore the `heroku db:pull`
command) I've had to look this up every time I want to import data.
Update bundled gems

I ran `bundle update` to get the latest versions of required gems.
Remove taps and sqlite3 gems

These were added in b460b3f6703c4317e130f4c54ecab668dca1757d, apparently so
that I could get 'taps' to work. I don't know whether I did have it working
then but I certainly can't get it to work now so I'm removing these from the
Gemfile.