* Remove allowTransparency attribute
`allowtransparency` is an Internet Explorer-only attribute that
controls the background transparency of an iFrame. When set to true,
it respects the background color of the iFrame. When set to false, it
sets the background color to that of the document.
This feature was removed in IE9 - falling out of React's support
commitment.
Developers that have somehow figured out how to get IE8 to work with
React 16.x can still use `allowtransparency="true"`, since React now
supports unrecognized attributes.
* Use correct attribute script location
* Use new UMD bundles in attribute fixtures
* Update attribute snapshot
* Blank for CI