As we've said many times here, the normal community understanding on contribution licensing is inbound=outbound, which is to say that contributors agree for their contributions to be licensed under the project's existing licence.

In the case of copyleft licences, as the linked question says, this is actually a requirement of the licence. In the case of the permissive licences like MIT, however, it's just a community understanding. Unless the project required a CLA from you, you could make an argument that you never licensed your contributions to the project at all, but I'd expect it to be a hard, uphill business to convince a judge of that (indeed, as Bart points out (thank you, Bart!), given GitHub's embedding of in=out in their TOS, it will likely be next to impossible).

But the project is completely entitled to change to a proprietary licence, and unless you can convince a judge that you never agreed to licence your contributions, you have no right to demand they stop using your contribution. One of the many advantages of copyleft licensing is that, once contributions have been accepted, the project can no longer unilaterally relicense. The permissive licences don't give the same protection, and this is generally understood, so what they've done, though not nice, is neither unlawful nor unethical.

One thing you can do is to take the copy of the MIT-licensed source you've found, and make sure it's available from your website (or at least, not solely from your github account). You have every right to do that, and as the search engines pick it up, you may hope that their proprietary-licensed version is supplanted by the free version.