docs: add jemoji plugin

This plugin enables jekyll to display emojis; the version added matches
current on github-pages (https://pages.github.com/versions/)

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/docs/Gemfile.lock b/docs/Gemfile.lock
index 4ec5c04..58b2529 100644
--- a/docs/Gemfile.lock
+++ b/docs/Gemfile.lock
@@ -255,6 +255,7 @@
 
 DEPENDENCIES
   github-pages
+  jemoji (~> 0.12.0)
   tzinfo-data
 
 BUNDLED WITH