Update Jekyll due to CVE

https://nvd.nist.gov/vuln/detail/CVE-2018-17567

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/docs/Gemfile b/docs/Gemfile
index 047e5e2..a9dad35 100644
--- a/docs/Gemfile
+++ b/docs/Gemfile
@@ -8,7 +8,7 @@
 #
 # This will help ensure the proper Jekyll version is running.
 # Happy Jekylling!
-gem "jekyll", "3.5.2"
+gem "jekyll", ">= 3.7.4"
 
 # This is the default theme for new Jekyll sites. You may change this to anything you like.
 # gem "jekyll-theme-cayman", "~> 0.1"