Upgrade lerna to 3.15

[leothekim]

In doing so, fixes this vulnerability:
https://github.com/trialspark/enzyme-context/network/alert/yarn.lock/handlebars/open

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[coveralls]

Coverage remained the same at 96.983% when pulling 6028876 on leo/upgrade-lerna into 998ae5d on master.

[minznerjosh]

Thanks!