Update development Ruby version to 2.5.9

Author: jgarber623

.ruby-version

@@ -1 +1 @@
-2.5.8
+2.5.9

CONTRIBUTING.md

@@ -8,9 +8,9 @@ There are a couple ways you can help improve indieweb-endpoints-ruby:
 
 ## Getting Started
 
-indieweb-endpoints-ruby is developed using Ruby 2.5.8 and is additionally tested against Ruby 2.6 and 2.7 using [Travis CI](https://travis-ci.com/indieweb/indieweb-endpoints-ruby).
+indieweb-endpoints-ruby is developed using Ruby 2.5.9 and is additionally tested against Ruby 2.6 and 2.7 using [Travis CI](https://travis-ci.com/indieweb/indieweb-endpoints-ruby).
 
-Before making changes to indieweb-endpoints-ruby, you'll want to install Ruby 2.5.8. It's recommended that you use a Ruby version managment tool like [rbenv](https://github.com/rbenv/rbenv), [chruby](https://github.com/postmodern/chruby), or [rvm](https://github.com/rvm/rvm). Once you've installed Ruby 2.5.8 using your method of choice, install the project's gems by running:
+Before making changes to indieweb-endpoints-ruby, you'll want to install Ruby 2.5.9. It's recommended that you use a Ruby version managment tool like [rbenv](https://github.com/rbenv/rbenv), [chruby](https://github.com/postmodern/chruby), or [rvm](https://github.com/rvm/rvm). Once you've installed Ruby 2.5.9 using your method of choice, install the project's gems by running:
 
 ```sh
 bundle install

README.md

@@ -18,7 +18,7 @@
 
 Before installing and using indieweb-endpoints-ruby, you'll want to have [Ruby](https://www.ruby-lang.org) 2.5 (or newer) installed. It's recommended that you use a Ruby version managment tool like [rbenv](https://github.com/rbenv/rbenv), [chruby](https://github.com/postmodern/chruby), or [rvm](https://github.com/rvm/rvm).
 
-indieweb-endpoints-ruby is developed using Ruby 2.5.8 and is additionally tested against Ruby 2.6 and 2.7 using [Travis CI](https://travis-ci.com/indieweb/indieweb-endpoints-ruby).
+indieweb-endpoints-ruby is developed using Ruby 2.5.9 and is additionally tested against Ruby 2.6, 2.7, and 3.0 using [Travis CI](https://travis-ci.com/indieweb/indieweb-endpoints-ruby).
 
 ## Installation