Blur up plugin is not compatible with IE

[slavanga]

The blur up plugin is using the find method which is not supported by IE.
https://github.com/aFarkas/lazysizes/blob/gh-pages/plugins/blur-up/ls.blur-up.js#L33

Error in IE:

Object doesn't support property or method 'find'

The error also occurs in the official demo for the plugin.
Until this is fixed the error can be prevented by including a polyfill for the find method.

[aFarkas]

Thank you!!!