package.json node engines config needs to be updated

[dustintownsend]

Now that FlexService supports multiple Node runtimes, the package.json engines config should be updated to reflect this. Currently it is set to node 6.x.x only, but 8.x and 10.x are now supported by Flex.

When using yarnpkg / yarn (instead of npm) it will fail when trying to install the kinvey-flex-sdk module.

Current:
"engines": { "node": "=6.x.x" }

Proposed:
"engines": { "node": "=6.x.x||=8.x.x||=10.x.x" }

If anyone is having this issue, the work around is to pass --ignore-engines argument to yarn.