Update mongoose to version 4.4.15 🚀

[greenkeeperio-bot]

Hello 👋

🚀🚀🚀

mongoose just published its new version 4.4.15, which is not covered by your current version range.

If this pull request passes your tests you can publish your software with the latest version of mongoose – otherwise use this branch to work on adaptions and fixes.

Happy fixing and merging 🌴

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The new version differs by 22 commits .

• 517442e chore: release 4.4.15
• f8c7e3a Merge pull request #4129 from Automattic/revert-4107-master
• 7495208 Revert "Apply plugins to schemas only once"
• 8e013c3 fix(schema): support overwriting array default
• 76ff26c test(schema): repro #4109
• e2f94a4 refactor(connection): use separate var statements
• dcbfcb4 docs: add waffle.io to home page
• e327138 fix(populate): assign values when resolving each populate
• 91c3eac test(populate): repro #4104
• 44cfd47 fix(aggregate): dont send async option to server
• 94e06a0 test(aggregate): repro #4101
• 746447a Merge branch 'master' of github.com:Automattic/mongoose
• 5ad5817 fix(model): ensure isNew set to false after insertMany
• 9dc5404 test(model): repro #4099
• 6549f08 Merge pull request #4107 from RobertWHurst/master

There are 22 commits in total. See the full diff.

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