spec/spec_helper.rb: Remove invalid config
spec/spec_helper.rb: Remove invalid config

Test harness jekyll configuration was specifying 'plugins'
as a directory, when there are no plugins and it must be
a list of plugins names on Jekyll 3.5+.

Fixes https://github.com/gjtorikian/jekyll-last-modified-at/issues/55

gjtorikian/jekyll-last-modified-at,gjtorikian/jekyll-last-modified-at