Prebid 2.0.0 Release

NOTE: The Prebid.js 2.0 release is a relatively small update that should affect few customers. The major version was updated because it is a 'breaking change', meaning that it could change system behavior for any user of Prebid.js benefiting from the limited bid caching feature, which is now turned off by default.

See #3278

🚀New Features

• New hooks API for 2.0 #3378
• switch default for useBidCache to false #3480
• allow globalVarName as babel plugin option #3491

🛠Maintenance

🐛Bug Fixes

• Bugfix - pbjs.getBidResponses #3492