Use an updated version of ruby

With this configuration, it always uses an updated version of ruby and bundler
is the one which manages version of the dependencies, hence working in all
environments. Before, dependencies could not be satisfied in updated
environments because of bundler being too new.

Gemfile.lock

@@ -1,51 +1,84 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    addressable (2.5.0)
-      public_suffix (~> 2.0, >= 2.0.2)
+    addressable (2.7.0)
+      public_suffix (>= 2.0.2, < 5.0)
     colorator (1.1.0)
-    ffi (1.9.17)
+    concurrent-ruby (1.1.5)
+    em-websocket (0.5.1)
+      eventmachine (>= 0.12.9)
+      http_parser.rb (~> 0.6.0)
+    eventmachine (1.2.7)
+    ffi (1.11.3)
     forwardable-extended (2.6.0)
-    jekyll (3.4.0)
+    http_parser.rb (0.6.0)
+    i18n (1.7.0)
+      concurrent-ruby (~> 1.0)
+    jekyll (4.0.0)
       addressable (~> 2.4)
       colorator (~> 1.0)
-      jekyll-sass-converter (~> 1.0)
-      jekyll-watch (~> 1.1)
-      kramdown (~> 1.3)
-      liquid (~> 3.0)
+      em-websocket (~> 0.5)
+      i18n (>= 0.9.5, < 2)
+      jekyll-sass-converter (~> 2.0)
+      jekyll-watch (~> 2.0)
+      kramdown (~> 2.1)
+      kramdown-parser-gfm (~> 1.0)
+      liquid (~> 4.0)
       mercenary (~> 0.3.3)
       pathutil (~> 0.9)
-      rouge (~> 1.7)
+      rouge (~> 3.0)
       safe_yaml (~> 1.0)
-    jekyll-sass-converter (1.5.0)
-      sass (~> 3.4)
-    jekyll-watch (1.5.0)
-      listen (~> 3.0, < 3.1)
-    kramdown (1.13.2)
-    liquid (3.0.6)
-    listen (3.0.8)
-      rb-fsevent (~> 0.9, >= 0.9.4)
-      rb-inotify (~> 0.9, >= 0.9.7)
+      terminal-table (~> 1.8)
+    jekyll-feed (0.13.0)
+      jekyll (>= 3.7, < 5.0)
+    jekyll-sass-converter (2.0.1)
+      sassc (> 2.0.1, < 3.0)
+    jekyll-seo-tag (2.6.1)
+      jekyll (>= 3.3, < 5.0)
+    jekyll-watch (2.2.1)
+      listen (~> 3.0)
+    kramdown (2.1.0)
+    kramdown-parser-gfm (1.1.0)
+      kramdown (~> 2.0)
+    liquid (4.0.3)
+    listen (3.2.1)
+      rb-fsevent (~> 0.10, >= 0.10.3)
+      rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
-    pathutil (0.14.0)
+    minima (2.5.1)
+      jekyll (>= 3.5, < 5.0)
+      jekyll-feed (~> 0.9)
+      jekyll-seo-tag (~> 2.1)
+    pathutil (0.16.2)
       forwardable-extended (~> 2.6)
-    public_suffix (2.0.5)
-    rb-fsevent (0.9.8)
-    rb-inotify (0.9.8)
-      ffi (>= 0.5.0)
-    rouge (1.11.1)
-    safe_yaml (1.0.4)
-    sass (3.4.23)
+    public_suffix (4.0.1)
+    rb-fsevent (0.10.3)
+    rb-inotify (0.10.0)
+      ffi (~> 1.0)
+    rouge (3.14.0)
+    safe_yaml (1.0.5)
+    sassc (2.2.1)
+      ffi (~> 1.9)
+    terminal-table (1.8.0)
+      unicode-display_width (~> 1.1, >= 1.1.1)
+    thread_safe (0.3.6)
+    tzinfo (1.2.5)
+      thread_safe (~> 0.1)
+    tzinfo-data (1.2019.3)
+      tzinfo (>= 1.0.0)
+    unicode-display_width (1.6.0)
+    wdm (0.1.1)
 
 PLATFORMS
   ruby
 
 DEPENDENCIES
-  jekyll (= 3.4.0)
+  jekyll (~> 4.0.0)
+  jekyll-feed (~> 0.12)
+  minima (~> 2.5)
+  tzinfo (~> 1.2)
   tzinfo-data
-
-RUBY VERSION
-   ruby 2.3.1p112
+  wdm (~> 0.1.1)
 
 BUNDLED WITH
-   1.13.6
+   2.0.2