Thanks to the efforts of @patrykkopycinski, Gatsby now supports React 15. This should be an easy upgrade for most people as long as your current site is showing deprecation warnings in the console.log. #252
@alampros also contributed a bug fix as he noticed that when setting the host option for gatsby develop and gatsby serve-build that the short version -h overrode the default help option. He changed that to -H so there would no longer be a conflict #247.