Update Gemfile.lock

Update jekyll version to 3.7.4 to remove security vulnerability CVE-2018-17567

Author: joshlk

Gemfile.lock

@@ -33,7 +33,7 @@ GEM
     github-pages (186)
       activesupport (= 4.2.10)
       github-pages-health-check (= 1.8.1)
-      jekyll (= 3.7.3)
+      jekyll (= 3.7.4)
       jekyll-avatar (= 0.5.0)
       jekyll-coffeescript (= 1.1.1)
       jekyll-commonmark-ghpages (= 0.1.5)