drop node 5 support
drop node 5 support

Summary:
Node 5 is unsupported, so we should stop worrying about it. Might also help
speed up Travis a bit.

Reviewed By: wincent

Differential Revision: D4816618

fbshipit-source-id: 4daa60ba98c5e9d84283bfbd001cb1250bed38cf

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