Upgrade us to Ruby 2.6.5 & Jekyll 4

CONTRIBUTING.md

@@ -21,13 +21,13 @@ advised that you use [RVM](https://rvm.io/rvm/install).
 Once you've got that installed and configured, get the latest version of Ruby with:
 
 ```bash
-$ rvm install 2.4.9
+$ rvm install 2.6.5
 ```
 
 Select that version and then create an isolated [gemset](https://rvm.io/gemsets/basics) with:
 
 ```bash
-$ rvm use 2.4.9
+$ rvm use 2.6.5
 $ rvm gemset create autonomic
 $ rvm gemset use autonomic
 ```
@@ -58,7 +58,7 @@ Edit content in the `.md` files under `src` and `src/_posts`.
 When you're hacking later on (or in a new shell), you'll need to run:
 
 ``` bash
-$ rvm use 2.4.8
+$ rvm use 2.6.5
 $ rvm gemset use autonomic
 ```