Update bin/readability to work with Ruby 3.x

These changes should be backwards compatible with older Ruby versions
according to the docs and testing it on Ruby 2.7.

Changes:
1. Use `require_relative` to avoid loading errors.
2. Use `URI.parse(...).read` instead of `open(...).read`
    - This seems to be backwards compatible with older Ruby versions.
        - Ex. https://www.rubydoc.info/stdlib/open-uri/2.3.1/OpenURI
    - The behavior should be identical to what we saw before.
    - `open(...).read` got deprecated in Ruby 3.0

README.md

@@ -28,7 +28,7 @@ Example
     require 'readability'
     require 'open-uri'
 
-    source = open('http://lab.arc90.com/experiments/readability/').read
+    source = URI.parse('http://lab.arc90.com/experiments/readability/').read
     puts Readability::Document.new(source).content
 
 

bin/readability

@@ -2,7 +2,7 @@
 require 'rubygems'
 require 'open-uri'
 require 'optparse'
-require File.dirname(__FILE__) + '/../lib/readability'
+require_relative '../lib/readability'
 
 options = { :debug => false, :images => false }
 options_parser = OptionParser.new do |opts|
@@ -28,7 +28,7 @@ if ARGV.length != 1
   exit 1
 end
 
-text = open(ARGV.first).read
+text = URI.parse(ARGV.first).read
 params = if options[:images]
   { :tags => %w[div p img a],
     :attributes => %w[src href],