Warn when reducer is not a function

[baibhavx]

const rootReducer = combineReducers({ 
  todos: todosReducer, 
  steps: {} // no warning, this is simply ignored 
})

• If the object supplied to combineReducers has a key whose value is not a function, it simply ignores it. With this fix, it gives a warning, letting the user know that each key in the object must point to a function.

[timdorr]

Needs at least one test case to go with the change.

[baibhavx]

@timdorr I added the tests to my commit but the build fails at `npm ci` with `zlib: unexpected end of file` and I have no idea why that is happening. If possible, please suggest how to make this pass. Thanks!

[timdorr]

This will probably be confusing by itself. I'd add more context to the message: The reducer for "${key}" is not a function

[baibhavx]

Good idea! I've updated the error message to The reducer for "${key}" is not a function format.

[timdorr]

Yeah, something is up with Travis. Not your fault, though! I'll see if I can figure it out...