Fix antigen theme integration.

Ugh. So, apparently I didn't dive deep enough into how antigen works (https://github.com/zsh-users/antigen/issues/78). It turns out using `antigen theme` is not necessary, and therefore the .zsh-theme proxy is not necessary either. If one uses `antigen bundle sindresorhus/pure` all works out as expected. I've asked the antigen authors to clarify their docs, but we can revert the proxy commit and update the docs. Sorry!