allow runners to submit large build logs

The default nginx configuration will only allow for 1 megabyte request bodies, so large build logs from runners will be rejected by the server.

This patch sets the limit to 10 megabytes. I'm not sure what a sensible default might be, but 1m seem a bit over-conservative to me.
Merge pull request #335 from bkw/patch-1

allow runners to submit large build logs