Update README.md with new rust version requirement
Update README.md with new rust version requirement

Neon requires Rust 1.18 since https://github.com/neon-bindings/neon/pull/231
rustbridge/neon,dherman/nanny,rustbridge/neon,rustbridge/neon,neon-bindings/neon,dherman/nanny,rustbridge/neon,neon-bindings/neon,rustbridge/neon,dherman/nanny,rustbridge/neon,neon-bindings/neon,neon-bindings/neon,dherman/nanny