Adjust gemspec to allow use with Jekyll 4.0

[zocoi]

Since jejyll 4 is released, bump up the requirement version

[ashmaroli]

@zocoi Couple of comments (FYI):

• Dependency "jekyll", "~> 4" means the next release can't be run with Jekyll 3.x. That would be bad as the majority of the users would still be using Jekyll 3.x series.
  Instead opt for "jekyll", ">= 3.3", "< 5.0"
• IMO, it'd be better if Travis would use the lowest common Ruby version which is rvm 2.4

[zocoi]

@ashmaroli Agree with the comment, I also updated node to 8 which the oldest LTS version.

Since jekyll 4 introduces new Rubocop configs and this gem inherits from those cops. I updated the codebase to clear all the offenses.

[tosbourn]

Just heard about this project and would love to see this change make it in so I can try jekyll-admin on my blog. 💜

[mertkahyaoglu]

@ashmaroli I think this is ready to go, right?

[ashmaroli]

I think this is ready to go

Not quite. First, I shall merge #497. Then reduce the scope of changes in this PR..

[mertkahyaoglu]

Good 👍 @zocoi We can merge this after conflicts are resolved then.

[ashmaroli]

@zocoi I've thinned out the changes made in this branch to keep the PR focused to the primary intention.
Regarding the use of Node 8 for testing, we can bump if necessary in the future. For now, tests run on it just fine.

[mertkahyaoglu]

Tested locally. Seems to be all good!

[ashmaroli]

Thank you for your contribution @zocoi 😃

[zocoi]

you're welcome, thanks for responding to this PR quickly :)