Update to use Go modules

Fixes: #109

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

vendor/github.com/sirupsen/logrus/writer.go

@@ -6,10 +6,16 @@ import (
 	"runtime"
 )
 
+// Writer at INFO level. See WriterLevel for details.
 func (logger *Logger) Writer() *io.PipeWriter {
 	return logger.WriterLevel(InfoLevel)
 }
 
+// WriterLevel returns an io.Writer that can be used to write arbitrary text to
+// the logger at the given log level. Each line written to the writer will be
+// printed in the usual way using formatters and hooks. The writer is part of an
+// io.Pipe and it is the callers responsibility to close the writer when done.
+// This can be used to override the standard library logger easily.
 func (logger *Logger) WriterLevel(level Level) *io.PipeWriter {
 	return NewEntry(logger).WriterLevel(level)
 }