Use UMD

The current module definition prelude is checking if a `define` function is present in scope. This suddenly caused breakage on a client site because a script that added a globlal define function was included on the page.

This PR fixes this by using the battle-tested UMD definition from https://github.com/ForbesLindesay/umd instead (which lets `module.exports` take precedence before a `define` function)

Side note: I had to lower the coverage threshold to 93 as I found no way to test the AMD/globals-branching in UMD.