fix(dev-tools) babel config overrides nesting

[chrisgervang]

For #427

Changes

• Babel overrides should be nested under overrides
• The overrides type can be an array or undefined. Babel throws ConfigError: .overrides must be an array, or undefined otherwise.

[ibgreen]

ConfigError: .overrides must be an array, or undefined otherwise.

My only concern is that I wonder why has this has not been a problem before?
Is this error new in a recent version of babel?

[chrisgervang]

Discussed with @Pessimistress, the api's intention is probably

overrides: {
  // global overrides
  overrides: [{}] // babel's overrides api
}

I'm going to change the docs instead