Flush nginx cache on deploy

[martinjobb]

Had an issue where I could not see my changes after deploy. Resolved it by flushing nginx cache in wp admin. But i think it should be a part of the release flow.

Added this to after-symlink.yml in my project. Maybe it should go into the boilerplate?

- name: Purge Nginx Cache
  shell: /bin/rm -rf /var/lib/nginx/fastcgi_cache/*