Build script broken by newer version of rubocop

[rdipardo]

Just reporting that I can locally reproduce the error output of this failed build with the following environment:

$ bundle list | awk '/slate /'
  * jekyll-theme-slate (0.1.1)
$ ruby -v
ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux]

The problem is that running script/bootstrap installs a recent version of rubocop:

$ bundle list | awk '/rubocop /'
  * rubocop (0.93.1)

The linting step in script/cibuild fails because rubocop has added some fussy new cops that aren't being filtered.
Similar to #39, this can be easily fixed by updating .rubocop.yml. Here's a config that works:

AllCops:
  NewCops: enable
  Exclude:
    - _site/**/*
Layout/LineLength:
  Enabled: false
Gemspec/RequiredRubyVersion:
  Enabled: false
Style/CaseLikeIf:
  Enabled: false

[rdipardo]

Fixed by efa1cd0