Make paper link point to PeerJ instead of PDF + update Gems

docs/Gemfile.lock

@@ -98,7 +98,7 @@ GEM
       gemoji (~> 2.0)
       html-pipeline (~> 2.2)
       jekyll (>= 3.0)
-    json (1.8.3)
+    json (2.1.0)
     kramdown (1.11.1)
     liquid (3.0.6)
     listen (3.0.6)

docs/_data/promo.yml

@@ -13,5 +13,5 @@
     href: docs/quick_start.html#python-api
     text: Get started in Python
   - type: button
-    href: static/prophet_paper_preprint.pdf
+    href: https://peerj.com/preprints/3190/
     text: Read the paper