use "timestamp" key instead of "time"

This is in accordance with the StackDriver documentation:

https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry

fixes #15

encoder.go

@@ -33,7 +33,7 @@ var logLevelSeverity = map[zapcore.Level]string{
 // encoderConfig is the default encoder configuration, slightly tweaked to use
 // the correct fields for Stackdriver to parse them.
 var encoderConfig = zapcore.EncoderConfig{
-	TimeKey:        "time",
+	TimeKey:        "timestamp",
 	LevelKey:       "severity",
 	NameKey:        "logger",
 	CallerKey:      "caller",