Clean out docs/gratipay each time we build it
Clean out docs/gratipay each time we build it

Cleaning it out is cheap, and prevents *.rst files from getting stuck in
there, which can lead to WARNINGs about missing modules.

gratipay/gratipay.com,gratipay/gratipay.com,gratipay/gratipay.com,gratipay/gratipay.com