Use math.div() for divisions

[polarbirke]

This fixes #5.

[strarsis]

@polarbirke: Thank you for this contribution!

Would it be possible to add this fix without breaking the compatibility with older implementations like libsass or ruby sass?

[polarbirke]

I've seen Bootstrap use a workaround: twbs/rfs#398 if you want to stay backwards-compatible 😄

[strarsis]

@polarbirke: If you could add a divide polyfill to this PR it would be great!

This wouldn't only fix the deprecation warnings and errors (when / division support is removed in future releases of Dart Sass), but also allow for older releases to still use this pow polyfill.

The documentation you add with this PR is very thorough and well-done by the way.

[strarsis]

@polarbirke: Merged #7. (Edit)
Could you also add a PR with your documentation about this, it isn't contained in this PR?
Thanks!

[polarbirke]

Ah, I intentionally left out the upgrading guide because now it's an "invisible" fix that doesn't change the interface or requirements for consumers.

What do you think should be documented?

[strarsis]

@polarbirke: You are right, this can be left it out.

[polarbirke]

Replaced by #7 (better solution without breaking change).