Upgrade to Ruby 2.5.8

.ruby-version

@@ -1 +1 @@
-2.5.3
+2.5.8

script/cibuild-docker

@@ -3,7 +3,7 @@
 set -ex
 
 # Set the ruby version in the Action definition matrix.
-: "${RUBY_VERSION:="2.5.3"}"
+: "${RUBY_VERSION:="2.5.8"}"
 
 docker build --build-arg "RUBY_VERSION=$RUBY_VERSION" -t github-pages .
 docker run --rm --workdir /src/gh/pages-gem github-pages script/cibuild