fix(package): require npm version 5.6.0 or higher

[ChristianMurphy]

npm <5.6.0 has issues with optional dependencies that are only installed for particular operating system. Causing dependencies such as fsevents to be added to the package-lock.json file on MacOS, but then be removed when a Linux system installs the project. npm >=5.6.0 includes a fix that normalizes handling of optional dependencies to allow for any OS to produce that same package-lock.json file from a given package.json

reference: https://github.com/npm/npm/blob/latest/CHANGELOG.md#v560-2017-11-27

to update to the latest version of npm

npm install --global npm@latest

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Contributor License Agreement adherence:

• [x] This Contribution is under the terms of Individual Contributor License Agreements (and also Corporate Contributor License Agreements to the extent applicable) appearing in the Apereo CLA roster.

[ChristianMurphy]

AppVeyor is flaking out again, error is unrelated to these changes.

npm ERR! path C:\projects\uportal-home\node_modules\requirejs\bin\r.js.1308609769
npm ERR! code EPERM
npm ERR! errno -4048
npm ERR! syscall rename
npm ERR! Error: EPERM: operation not permitted, rename 'C:\projects\uportal-home\node_modules\requirejs\bin\r.js.1308609769' -> 'C:\projects\uportal-home\node_modules\requirejs\bin\r.js'

https://ci.appveyor.com/project/ChristianMurphy/uportal-home/build/1.0.136#L28

edit Flakiness will likely be resolved by https://github.com/uPortal-Project/uportal-app-framework/issues/346 edit 2 short term workaround https://github.com/uPortal-Project/uportal-home/pull/757