downgrade Psych for DisallowedClass errors on Time

  $ bundle exec rake
  Psych::DisallowedClass: Could not load blog entry file "content/*.readably"
  Tried to load unspecified class: Time

See also ruby/psych#362

Gemfile

@@ -4,6 +4,7 @@ source 'https://rubygems.org'
 gem 'rake', '~> 13'
 gem 'slim', '~> 4'
 gem 'sass', '~> 3'
+gem 'psych', '~> 2'
 
 # optional
 group :markdown do

Gemfile.lock

@@ -2,6 +2,7 @@ GEM
   remote: https://rubygems.org/
   specs:
     ffi (1.15.5)
+    psych (2.2.4)
     rake (13.0.6)
     rb-fsevent (0.11.1)
     rb-inotify (0.10.1)
@@ -23,11 +24,12 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
+  psych (~> 2)
   rake (~> 13)
   redcarpet (~> 3)
   rouge (~> 3)
   sass (~> 3)
   slim (~> 4)
 
 BUNDLED WITH
-   2.2.17
+   2.3.23

Rakefile

@@ -16,7 +16,7 @@ task :style
 # helper libraries
 #-----------------------------------------------------------------------------
 
-require 'yaml'
+require 'psych'
 require 'time'
 require 'uri'
 
@@ -294,7 +294,7 @@ end
 notify :load, @config_file
 
 begin
-  @config = YAML.load_file(@config_file).to_hash
+  @config = Psych.load_file(@config_file).to_hash
 rescue => error
   error.message.insert 0,
     "Could not load blog configuration file #{@config_file.inspect}\n"
@@ -317,7 +317,7 @@ notify :load, @entry_source_files_glob
 entry_sources_by_output = Hash.new {|h,k| h[k] = [] }
 @entries = Dir[@entry_source_files_glob].map do |source_file|
   begin
-    entry = YAML.load_file(source_file)
+    entry = Psych.load_file(source_file)
     entry[:source_file] = source_file
 
     sanitized_file = sanitize_filename(source_file)