Using 'value' rather than 'nodeValue'.

Started getting a deprecation error in Chrome with regards to nodeValue:

```'Attr.nodeValue' is deprecated. Please use 'value' instead. ```

So I changed it and I no longer get the warning, but does anyone know if this will bork older browsers?

Here's my todo list for the PR:
- [x] Replace nodeValue with value
- [ ] Bump version and rebuild

Didn't want to do a build as I don't know how you handle the versions and such.

Fixes https://github.com/patrickkunka/mixitup/issues/159