Fix linting errors & fix GitHub asset URL

.travis.yml

@@ -1,7 +1,6 @@
 language: ruby
 cache: bundler
-sudo: false
-rvm: 2.5
+rvm: 2.6
 
 install: script/bootstrap
 script: script/cibuild

Gemfile

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 source 'https://rubygems.org'
 
 gemspec

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
 

jekyll-theme-hacker.gemspec

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 Gem::Specification.new do |s|
   s.name          = 'jekyll-theme-hacker'
   s.version       = '0.1.1'

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-hacker.gemspec

script/validate-html

@@ -1,4 +1,5 @@
 #!/usr/bin/env ruby
+# frozen_string_literal: true
 
 require 'w3c_validators'