Update .gemspec for jekyll 4.0.0

[zerobase]

Hi, @ysk24ok. I have updated the gemspec dependency of Jekyll from "~> 3.5" to ">= 3.5" and I confirmed it works well with Jekyll 4.0.0. Thanks.

[ysk24ok]

Thank you for this PR, but there is a conflict because #11 is merged.
It would be great if you resolve the conflict.

In addition, how about setting a upper bound, e.x. ">= 3.8.6", "< 4.1" ?
Without upper bound, some users may suffer if new major or minor version such as 5.0.0 or 4.1.0 doesn't work with this plugin properly.
So setting an upper bound of version is a good idea.

[zerobase]

@ysk24ok okay, I have installed Appraisal to test this gem with Jekyll 3.5.0 and 4.0.0. Then it is reasonable to set the Jekyll dependency ">= 3.5", "< 4.1" isn't it?

[ysk24ok]

Appraisal is new to me.
Maybe we can drop support for 3.5-3.7.
Anyway thank you for the PR!