Update dependency wpackagist-theme/twentyfifteen to v4

This MR contains the following updates:

Package	Type	Update	Change
wpackagist-theme/twentyfifteen (source)	require	major	3.9 -> 4.0

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This MR has been generated by Renovate Bot.

mentioned in issue #139

added 3 commits

• 05b8d640...1e839aaa - 2 commits from branch master
• 918e9d89 - Update dependency wpackagist-theme/twentyfifteen to v4

Compare with previous version

enabled an automatic merge when all merge checks for 918e9d89 pass

merged

mentioned in commit c3068a0f