Fix htmlproofer configuration for local pages

Based on gjtorikian/html-proofer#170 (comment)

Author: julienrf

.github/workflows/ci.yml

@@ -34,4 +34,5 @@ jobs:
              --ignore-empty-alt=true\
              --allow-hash-href=true\
              --enforce-https=false\
-             --ignore-missing-alt=true
+             --ignore-missing-alt=true\
+             --swap-urls "https\:\/\/scala\.epfl\.ch:"

_config.yml

@@ -5,7 +5,7 @@ description: > # this means to ignore newlines until "baseurl:"
   The Scala Center at EPFL. Not-for-profit, "for the good of all"
   steward of Scala.
 baseurl: "" # the subpath of your site, e.g. /blog
-url: "http://scala.epfl.ch" # the base hostname & protocol for your site
+url: "https://scala.epfl.ch" # the base hostname & protocol for your site
 twitter_username: scala_lang
 github_username:  scala