Change the way `marked` is imported to keep IE11 support

[tjrobinson]

marked 0.7.0 has a breaking change for IE11: https://github.com/markedjs/marked/issues/1585

I believe the fix for angular-marked, if it's being used with marked 0.7.0, is to change this line:

https://github.com/Hypercubed/angular-marked/blob/c10be543da2cea906230efe2f16fd73f8a96d21d/lib/angular-marked.js#L191

To: m = require('marked/lib/marked')