update Caddy to v0.10.13 (#2154)

Update Caddy to v0.10.13 so that environment variables may be used after
Go template (e.g. in template's answer). v0.11.0 has been released, but
v0.10.13 is a smaller set of changes and has the necessary fix.

Fixes #2153

Makefile

@@ -29,7 +29,7 @@ godeps:
 	go get -u github.com/miekg/dns
 	go get -u github.com/prometheus/client_golang/prometheus/promhttp
 	go get -u github.com/prometheus/client_golang/prometheus
-	(cd $(GOPATH)/src/github.com/mholt/caddy              && git checkout -q v0.10.11)
+	(cd $(GOPATH)/src/github.com/mholt/caddy              && git checkout -q v0.10.13)
 	(cd $(GOPATH)/src/github.com/miekg/dns                && git checkout -q v1.0.12)
 	(cd $(GOPATH)/src/github.com/prometheus/client_golang && git checkout -q v0.8.0)
 	@ # for travis only, if this fails we don't care, but don't see benchmarks