Promise polyfill required as of v1.3.0

[techfg]

Describe the bug
The changes introduced to support UMD in #314 broke compatibility with ES5 as ES5 does not support native Promises. While the majority of targeted devices/browers now support ES6 (which includes Promises), this is a breaking change and IM should provide a path to support ES5.

When IM is refactored/rewritten to ES6 module, a transpiler will eliminate the requirement for a separate dependency to be loaded. For now, if IM needs to be used in an ES5 environment, a Promise polyfill is required to be loaded.

To Reproduce
Steps to reproduce the behavior:

1. Open USA example in non-ES6 browser (e.g. IE 11)
2. Exception encountered: SCRIPT5009: 'Promise' is undefined

Expected behavior
IM should work in ES5 devices/browsers.

Screenshots
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Additional context
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