Bootstrap 3.3.7 - fatal error: parse error: failed at `&:extend(.clearfix all);

[Casper-O]

It seems to me your repo is the most maintained, of all the Forks of the Original Repo.

I have tested with your 5.1 release that results in the following error:
fatal error: parse error: failed at&:extend(.clearfix all);less/bootstrap/mixins/grid.less on line 11