Editor broken on IE11

[tellthemachines]

Describe the bug

Gutenberg is not rendering at all on IE11. I traced the breakage to this commit; the error messages point to the problem being with the newly-added hex-rgb package using template literals.

To reproduce
Steps to reproduce the behavior:

1. Using IE, navigate to "new post" page.
2. Verify that page remains blank after loading, and errors appear in the browser console.

Expected behavior
Expected the post editor to render normally.

Desktop (please complete the following information):

• OS: Windows 7 - 10
• Browser: IE11
• Version: all versions