Change turbolinks to different version.

After upgrading turbolinks in a previous commit, it broke scrollspy.
Investigating, it turns out that commit
561c130eac7f8af886bc07d1bea12389bcb13d6b [1] is to blame. So, I've
forked turbolinks, removed the offending commit, and changed the Gemfile
to use my version instead.

1.
https://github.com/rails/turbolinks/commit/561c130eac7f8af886bc07d1bea12389bcb13d6b