Use platforms instead of install_if

[dleidert]

With :install_if bundler will check for the gems even if they are not required
on our platform. To prevent users from having this issue use :platforms
instead: https://lists.debian.org/debian-ruby/2020/04/msg00062.html

fixes #8139

[DirtyF]

LGTM

[ashmaroli]

@dleidert We also have a similar usage in our dev Gemfile. Could you patch those lines as well..?
Thanks.

jekyll/Gemfile

Lines 83 to 88 in 2bc1e04

	# Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem
	# and associated library
	install_if -> { RUBY_PLATFORM =~ %r!mingw|mswin|java! } do
	gem "tzinfo", "~> 1.2"
	gem "tzinfo-data"
	end

[ashmaroli]

@DirtyF Do you think this patch qualifies to be backported for v4.0.x..?

[DirtyF]

@ashmaroli Not if we release 4.1 sooner than later. As you see fit.

[DirtyF]

@jekyll: merge +fix