Fix commonjs exports

[vovkasm]

Fixed: #1266 and facebook/react-native#22863

[vovkasm]

@bvaughn Take attention please. It would be also very helpful to have releases tagged in git.

[vovkasm]

Also same issue: #1265

[vovkasm]

Also the tests passed I'm not sure is it right fix in common. Yes it fix RN issue, but should this module be available to direct inclusion from browser environment? May be better solution to split builds and make distinct builds for various targets (node, browser, es modules) and set appropriate entry points in package.json?
What people think? If this is the case, I can rework patch.

[bvaughn]

Hm, it seems like this was something I missed/broke when upgrading Webpack 1 -> 4 (PR #1235) and we're just noticing it now because we haven't released react-devtools-core since then.

The libraryTarget used to be "umd" before that change.

[bvaughn]

Thanks for helping track down the root cause of this!

I'm going to restore the value to "umd" to be safest.

[bvaughn]

Just published v3.5.1 with the "umd" change– which seems to fix this issue. So thanks again!