fix: avoid any for oneOf arrays with identifiers

[unstubbable]

Avoid anys in union types that are generated for oneOf with an array of identifiers, e.g.:

const FOO = 'foo';
const BAR = 'bar';
const fooOrBar = [FOO, BAR];

export default class SomeComponent extends React.Component {
   static propTypes = {
     someOneOf: React.PropTypes.oneOf(fooOrBar)
   };
}

now results in:

export type SomeComponentSomeOneOf = "foo" | "bar";

export interface SomeComponentProps {
  someOneOf?: SomeComponentSomeOneOf;
}

instead of:

export type SomeComponentSomeOneOf = any | any;

[KnisterPeter]

LGTM