server/play: Use golang.org/x/net/context/ctxhttp package

The issue golang/go#21358 has been resolved upstream, so there's
no more need to use a temporary fork of the ctxhttp package.

Also, github.com/shurcooL/go/ctxhttp has been deprecated and will
be deleted, see shurcooL/go@58262d1.

server/play/get.go

@@ -15,7 +15,7 @@ import (
 	"github.com/dave/services/getter/get"
 	"github.com/dave/services/getter/gettermsg"
 	"github.com/dave/services/session"
-	"github.com/shurcooL/go/ctxhttp"
+	"golang.org/x/net/context/ctxhttp"
 	"gopkg.in/src-d/go-billy.v4"
 )