Test on multiple React versions

I’d like to run our tests on multiple versions of React in CI, if we can.

• Travis supports build matrices which we can use to run multiple jobs with different environment variables, so that should help.
• npm supports a --prefix config option that we can use to change (I believe) the location where it looks for package.json and node_modules.

So we may be able to use these 2 features in combination to do something like this in our Travis config:

env:
  - NPM_PREFIX=react-15
  - NPM_PREFIX=react-16
install:
  - npm install --prefix=$NPM_PREFIX

It seems a little hacky, and there would probably be some bugs to work out. But it might work.

[cellog]

Quick chime in: this was the only way I found to do it that didn’t have the npm equivalent of dll hell. If the way you implement it is similar, let me know, I would be willing to abstract that into an external package for testing against multiple versions of external deps

[timdorr]

We’ve got this set up on React Redux if you want to steal some ideas: https://github.com/reduxjs/react-redux/tree/master/test @cellog set it up for us.