Migrate to React 15.5 to make 16.0 migration easier

[TristanWright]

https://facebook.github.io/react/blog/2017/04/07/react-v15.5.0.html

So while the warnings may cause frustration in the short-term, we believe prodding developers to migrate their codebases now prevents greater frustration in the future. Proactively fixing warnings ensures you are prepared for the next major release. If your app produces zero warnings in 15.5, it should continue to work in 16 without any changes.

(Emphasis mine)