Polyfill create_function

[TimBroddin]

Polyfilling create_function in WPCOMSH should fix some sites.

See: https://github.com/php5friends/polyfill-create_function

[obenland]

One of the things to consider with this is that we'll have to keep that polyfill forever.

We wouldn't know which sites it effects (including sites that become dependent on it after we merge it), so removing it would be very painful.

Not necessarily a blocker, but a circumstance we should acknowledge if this is what we decide to do.