Add Korean (ko_KR) translation.

Props Josh Kim.
Fixes https://github.com/TobiasBg/TablePress/pull/27.
Clean up version numbers and outdated strings in the *.po files.
Update Grunt Autoprefixer module version to 2.0.0.
Travis CI: Test on WordPress 4.0.1 which is now the latest stable release.
Update "Tested up to" information for WordPress 4.0.1.
Update to CodeMirror 4.8.0.
Update the Chinese (Taiwan) translation for TablePress 1.5.1.