Update Rakefile

Trying to serve jekyll without `bundle exec` throws an error (jekyll/jekyll#3084)

Author: yourwebmaker

Rakefile

@@ -16,7 +16,7 @@ CONFIG = {
 
 desc 'Preview the site with Jekyll'
 task :preview do
-    sh "jekyll serve --watch --drafts --baseurl '' --config _config.yml,_config-dev.yml"
+    sh "bundle exec jekyll serve --watch --drafts --baseurl '' --config _config.yml,_config-dev.yml"
 end
 
 desc 'Search site and print specific deprecation warnings'
@@ -85,4 +85,4 @@ task :page do
     post.puts 'description: ""'
     post.puts "---"
   end
-end # task :page
+end # task :page