Fix a rubocop failure.
Revert "Merge branch 'vendor_keyframes'"

This reverts commit e6c60c4946d3ecd11996175d9a35b5fe5a3374b1, reversing
changes made to 767c53f7af6057e4ec75c22075edd8846229d524.
Revert "Actually parse @keyframes directives."

This reverts commit f4b07be31558b755371fa426829c9f3188f6a19a.

Conflicts:
	doc-src/SASS_CHANGELOG.md
Merge branch 'revert-keyframes'

The parsing changes to @keyframes, while on the whole good, broke mixins that
included keyframes blocks. We'll figure out a better way to do this after 3.3.

See #946
Closes #1102