Update plugin key name

[tleunen]

Ref #74.

Need to investigate why the beta.2 works, but not the beta.31 (the latest)

[fatfisz]

@tleunen Just to make sure: you meant beta.3?

I wandered in the wrong direction for a bit, but I think I understand what's happened now.

Basically the signature changed here: https://github.com/babel/babel/pull/6350/files#diff-5fbe3f3548c58af9b3a1edd90daf9035L12, and the change to the Plugin class itself can be seen here: https://github.com/babel/babel/pull/6350/files#diff-87e2d4f5f08732f1924207a97475a26c

Instead of an array of pairs (arrays) consisting of the plugin info and options, the options are integrated into the plugin info. I'll prepare a patch for that (and a bit of an extra 😉).

[fatfisz]

I created another PR with the changes, as I couldn't modify this one.