Update gemspec file to recent styles

[mishina2228]

This PR updates yajl-ruby.gemspec to recent styles. Here are the details:

• date and rubygems_version should not be set
  • RubyGems says not to set the value to date and rubygems_version.
• Remove deprecated test_files
  • Please see the following for details.
    • Documentation about 'test_files' is confusing rubygems/guides#90
    • Don't package test files with gem rubygems/bundler#3207
• Update homepage to https
  • This reduces unnecessary redirection from http://... to https://... when someone visits this gem's homepage via https://rubygems.org/gems/yajl-ruby or some tools or APIs that use the gem's metadata.

[jhawthorn]

Thanks!