Doesn't work in IE

[btecu]

On release 6.0.1, using the helper version:

{{on-key "cmd+o" this.someAction}}

When pressing any key, it throws an error:

TypeError: Object doesn't support property or method 'from'

[lukemelia]

@btecu ember-keyboard 6 drops support for IE. I'll make sure this gets documented in the README.

[lukemelia]

Updated README in #285