Reflect dual licensed vis.js in README

Further to #285, it seems the `README.md` wasn't changed in https://github.com/AlexVangelov/vis/commit/124395d9746cd963ddf4db2500c630c7c89bb7aa to reflect the dual license status.
Merge pull request #486 from mdxs/patch-2

Reflect dual licensed vis.js in README