Update peerDependencies for react > 16

Our team loves this library and we are using v1.0.0 without any issues in a project with react and react-dom ~16.6.0.

However, when installing the package, we are getting warnings because the peerDependencies in package.json is still set to versions of react and react-dom below 16:

react-codemirror/package.json

Lines 32 to 33 in c3ae528

    
           "react": ">=15.5 <16", 
        
           "react-dom": ">=15.5 <16"

It looks like the < 16 restriction was set some time ago in https://github.com/JedWatson/react-codemirror/pull/64/files#diff-b9cfc7f2cdf78a7f4b91a753d10865a2R30 . Is it possible that the library works just fine with react and react-dom > 16, or are there known issues with that version that I have not happened to encounter yet?

If there are no known issues, any objections to opening a PR to adjust the peer dependencies?

@mikeboyle This lib is pretty much abandoned. Check out react-codemirror2.