Update include_relative tests

This verifies you can’t go “up” in the include
Add some more docs on `include_relative`
Set tag name as ivar

This lets us use just one syntax_example method for both classes.
Fixing "vertycal" typo

s/vertycal/vertical

Unless, like, "vertycal" was on purpose or something.
Merge pull request #2889 from pdehaan/patch-1
Update history to reflect merge of #2889 [ci skip]
Merge pull request #2884 from gjtorikian/improve-docs-for-include-relative
Update history to reflect merge of #2884 [ci skip]
Give script/proof the ability to be force-run. [ci skip]
Allow use of script/cibuild outside Travis. [ci skip]
Add push, pop, shift, and unshift Liquid filters.

Fixes #2892.
Release Jekyll :gem: v2.4.0
Merge pull request #2895 from jekyll/array-filters
Update history to reflect merge of #2895 [ci skip]
Merge pull request #2896 from jekyll/release-2-4

Release Jekyll :gem: v2.4.0
Add #2895 to release post.

#2896.
Release :gem: 2.3.0
Release :gem: 2.4.0
Update site history.
Fix link to #2895.