Ruby 2.7 warnings

[dctucker]

Not urgent, but since extras hasn't seen an update in quite some time, it might be nice to make it a little less noisy when doing a jekyll build using Ruby 2.7.

$ bundle exec jekyll build
.../vendor/gems/ruby/2.7.0/gems/extras-0.3.0/lib/extras/string.rb:12: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call

Related issue: envygeeks/jekyll-assets#649