[chore] don't test on node versions < 6.x.y.

[diasbruno]

Don't test on node versions < 6.x.y.

Acceptance Checklist:

• All commits have been squashed to one.
• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[coveralls]

Coverage decreased (-13.4%) to 71.429% when pulling 4c6d40e on diasbruno:chore/dont-tests-on-node-lt-6 into 20fcdc3 on reactjs:master.