Raise error if pandoc does not return success (requires ruby 1.9.3+)

At the moment if pandoc fails (e.g. you pass invalid options) then pandoc-ruby silently returns an empty string with no indication of the error. This patch detects the return code and raises an error if it is non-zero.

~~~
$ irb20 -Ilib
irb(main):001:0> require 'pandoc-ruby'
=> true
irb(main):002:0> PandocRuby.new("# hello", "badopt").to_html5
RuntimeError: pandoc: unrecognized option `--badopt'
Try pandoc --help for more information.

	from /Users/brian/git/pandoc-ruby/lib/pandoc-ruby.rb:219:in `execute'
	from /Users/brian/git/pandoc-ruby/lib/pandoc-ruby.rb:201:in `convert_string'
	from /Users/brian/git/pandoc-ruby/lib/pandoc-ruby.rb:135:in `convert'
	from /Users/brian/git/pandoc-ruby/lib/pandoc-ruby.rb:169:in `block (2 levels) in <class:PandocRuby>'
	from (irb):2
	from /usr/local/bin/irb20:12:in `<main>'
~~~