diff --git a/.travis.yml b/.travis.yml
index 0c7c8ed7..a871f2a9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,6 @@
 language: ruby
 cache: bundler
-sudo: false
-rvm: 2.5
+rvm: 2.6
 
 install: script/bootstrap
 script: script/cibuild
diff --git a/Gemfile b/Gemfile
index fa75df15..7f4f5e95 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 source 'https://rubygems.org'
 
 gemspec
diff --git a/index.md b/index.md
index 4ea57371..86110519 100644
--- a/index.md
+++ b/index.md
@@ -94,7 +94,7 @@ end
 
 ### Small image
 
-![Octocat](https://assets-cdn.github.com/images/icons/emoji/octocat.png)
+![Octocat](https://github.githubassets.com/images/icons/emoji/octocat.png)
 
 ### Large image
 
diff --git a/jekyll-theme-primer.gemspec b/jekyll-theme-primer.gemspec
index 78871117..4d1c9a6b 100644
--- a/jekyll-theme-primer.gemspec
+++ b/jekyll-theme-primer.gemspec
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 Gem::Specification.new do |s|
   s.name          = 'jekyll-theme-primer'
   s.version       = '0.5.3'
diff --git a/script/cibuild b/script/cibuild
index b935e7fa..fa3a88ff 100755
--- a/script/cibuild
+++ b/script/cibuild
@@ -3,7 +3,7 @@
 set -e
 
 bundle exec jekyll build
-bundle exec htmlproofer ./_site --check-html --check-sri --disable-external
+bundle exec htmlproofer ./_site --check-html --check-sri
 bundle exec rubocop -D
 bundle exec script/validate-html
 gem build jekyll-theme-primer.gemspec
diff --git a/script/validate-html b/script/validate-html
index 801c00fa..d40c3dab 100755
--- a/script/validate-html
+++ b/script/validate-html
@@ -1,4 +1,5 @@
 #!/usr/bin/env ruby
+# frozen_string_literal: true
 
 require 'w3c_validators'