Lock psych to v4.x

Refer Ruby comment for details.

Author: ashmaroli

Gemfile

@@ -5,4 +5,12 @@ gemspec
 
 gem "jekyll", ENV["JEKYLL_VERSION"] if ENV["JEKYLL_VERSION"]
 gem "kramdown-parser-gfm" if ENV["JEKYLL_VERSION"] == "~> 3.9"
+
+# Psych 5 has stopped bundling `libyaml` and expects it to be installed on the host system prior to
+# being invoked.
+# Since we don't have a direct dependency on the Psych gem (it gets included in the gem bundle as a
+# dependency of the `rdoc` gem), lock psych gem to v4.x instead of installing `libyaml` in our
+# development / CI environment.
+gem "psych", "~> 4.0"
+
 gem "test-unit"