Add autoprefixer-core to package.json

[vsn4ik]

Reproduce issue:

rm -rf ./node_modules
npm install
grunt

[jonrohan]

Whoops. We missed adding autoprefixer-core. Thanks @vsn4ik! <3

Before we can merge your changes, we need you to review our Contributor License Agreement (CLA). We think of CLAs as an open source software license between you and the project, letting us and other users know that we have permission to use the code you contribute.

At the bottom of the pull request, you'll see the note about a failed test. Just click the Details link, review the CLA, and sign off at the bottom. Let me know if you have any questions.

[mdo]

Thanks @vsn4ik!