External Libraries: Update clipboard.js to 2.0.6.

This change includes some bug fixes and removal of the PhantomJS dependency.

Changelog: https://github.com/zenorocha/clipboard.js/compare/v2.0.4...v2.0.6

Props hareesh-pillai.
Fixes #50306.
Built from https://develop.svn.wordpress.org/trunk@47895


git-svn-id: https://core.svn.wordpress.org/trunk@47669 1a063a9b-81f0-0310-95a4-ce76da25c4cd