Dependencies: bump Rubocop to 0.5.1

[DirtyF]

I forgot to do that in #4 😊

@benbalter Just to know what actions to take after a Jekyll release, what would you recommend for dependencies bump in plugins?

• Should plugins follow the latest Jekyll version? (We bumped Rubocop to 0.5.1 in 3.6.2)
• Should we allow bundler to get latest patch releases? (e.g, 0.5 instead of 0.5.1 here)

[benbalter]

@DirtyF if plugins have

inherit_gem:
  jekyll: .rubocop.yml

My personal 2 cents is to follow the latest stable Jekyll within whatever pessimistic constraints its gemspec defines, meaning I'm fine if a plugin gets a patch release, but am not strongly opinionated and defer to you as long as we're consistent / it doesn't cause headaches or more work.

[stale]

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