Update dependency jekyll to v4

Gemfile.lock

@@ -21,7 +21,7 @@ GEM
     coffee-script-source (1.12.2)
     colorator (1.1.0)
     commonmarker (0.23.10)
-    concurrent-ruby (1.2.3)
+    concurrent-ruby (1.3.1)
     connection_pool (2.4.1)
     dnsruby (1.70.0)
       simpleidn (~> 0.2.1)
@@ -96,7 +96,7 @@ GEM
       activesupport (>= 2)
       nokogiri (>= 1.4)
     http_parser.rb (0.8.0)
-    i18n (1.14.1)
+    i18n (1.14.5)
       concurrent-ruby (~> 1.0)
     jekyll (3.9.5)
       addressable (~> 2.4)
@@ -234,12 +234,13 @@ GEM
       sawyer (~> 0.9)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
-    public_suffix (5.0.4)
+    public_suffix (5.0.5)
     racc (1.7.3)
     rb-fsevent (0.11.2)
-    rb-inotify (0.10.1)
+    rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rexml (3.2.6)
+    rexml (3.2.8)
+      strscan (>= 3.0.9)
     rouge (3.30.0)
     ruby2_keywords (0.0.5)
     rubyzip (2.3.2)
@@ -254,6 +255,7 @@ GEM
       faraday (>= 0.17.3, < 3)
     simpleidn (0.2.1)
       unf (~> 0.1.4)
+    strscan (3.1.0)
     terminal-table (1.8.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     typhoeus (1.4.1)