Add workaround for certificate-related issues

Author: anishathalye

.ci/cert-workaround

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+
+mkdir -p /usr/local/share/ca-certificates/extra
+cd /usr/local/share/ca-certificates/extra
+
+# Manually get intermediate CA certificates to deal with websites that don't
+# serve a full certificate chain. These are misconfigured websites, but we want
+# to be able to handle them anyways.
+wget https://letsencrypt.org/certs/lets-encrypt-x3-cross-signed.pem.txt -O lets-encrypt-x3-cross-signed.crt
+wget https://spaces.at.internet2.edu/download/attachments/24576265/InCommonRSAServerCA_2.crt
+
+# Remove expired root certificates to work around OpenSSL 1.0.x issue.
+sed -i 's/^\(mozilla\/AddTrust_External_Root.crt\)$/!\1/' /etc/ca-certificates.conf
+
+update-ca-certificates

.travis.yml

@@ -1,12 +1,15 @@
 language: ruby
+dist: bionic
+
 before_install:
   - gem update --system # necessary, see https://bundler.io/blog/2019/05/14/solutions-for-cant-find-gem-bundler-with-executable-bundle.html
+  - sudo .ci/cert-workaround
 before_script:
   - gem install html-proofer
 script:
   - bundle exec jekyll build
   - >-
-    htmlproofer ./_site
+    travis_retry htmlproofer ./_site
     --check-html
     --check-favicon
     --enforce-https