Update/remove README special thanks section

[MasterOdin]

The special-thanks section is out of date and features two things that slate does not use anymore (jquery.tocify.js, middleman-gh-pages) and then leaves out a whole bunch of other things slate depends upon (rouge, middleman-sprockets, jquery, lunr, etc.).

Should this section be removed or updated to feature the current full list of first-party dependencies (i.e. the ones listed in the Gemfile and the vendored JS dependencies).

[MikeRalphson]

:+1: for removal. Dependencies are documented (especially if we go down the route of adding a dummy package.json), let's reserve thanks for contributors and sponsors.

[lord]

Also +1 for removal