Add support for checking YAML validity using Travis CI

.travis.yml

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+language: ruby
+rvm:
+    - 2.2

Gemfile

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+group :test do
+    gem 'rake'
+end

Rakefile

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+task default: %w[test]
+
+task :test do
+	  ruby "tests/yaml.rb"
+end

tests/yaml.rb

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+#!/usr/bin/env ruby
+require 'yaml';
+$file_count = 0;
+markdown_files = Dir["./**/*.html.markdown"]
+markdown_files.each do |file|
+	begin
+		YAML.load_file(file)
+		$file_count = $file_count + 1
+	rescue Exception => msg
+		puts msg	
+	end
+end
+files_failed = markdown_files.length - $file_count
+if files_failed != 0
+	puts "FAILURE!!! #{files_failed} files were unable to be parsed!"
+	puts "Please check the YAML headers for the documents that failed!"
+	exit 1
+else
+	puts "Success. All #{$file_count} were checked"
+	exit 0
+end