Add expect to devDependencies to run tests

[jiayihu]

expect package is missing from devDependencies and it's required to run tests in this example.

[gaearon]

Are any other examples missing dependencies? Can you delete node_modules in the root directory and verify that all other examples work fine with just their own node_modules?

[jiayihu]

I didn't have node_modules installed in root because I was interested only in the examples :D Anyway I checked npm start and npm test whenever possible, tried also to play around with all the examples and none of them report errors in terminal or browser console ✅

[gaearon]

Thank you!