Object.assign - version 0.9

[dbabaioff]

The prebid code is using from version 0.9 (#349) Object.assign() - this ES6 function is not supported on IE and some mobile browsers.
To solve this issue I see two options: include a polyfill to make Object.assing work on older browsers or create a simple extend function and use it instead the Object.assign.

[mkendall07]

@dbabaioff
Thanks for pointing this out. We will use https://babeljs.io/docs/plugins/transform-object-assign/ to polyfill.

[mkendall07]

#369