This is a BREAKING CHANGE.
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### Platforms affected
n/a - development
### What does this PR do?
Drop Node.js v4 support and bumps the CI images to Node.js v6
### What testing has been done on this change?
TravisCI and Appveyor test results.
### Checklist
- [ ] [Reported an issue](http://cordova.apache.org/contribute/issues.html) in the JIRA database
- [ ] Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
- [ ] Added automated test coverage as appropriate for this change.
