Updates to lumberjack package released 4 years ago (#6)

This updates to gopkg.in/natefinch/lumberjack.v2 which was released in
mid 2017. This and other tidies result in less dependency declarations
in downstream projects.

For example:
```diff
--- a/go.mod
+++ b/go.mod
@@ -20,7 +20,6 @@ require (
        github.com/mattn/go-sqlite3 v1.14.6
        github.com/mitchellh/go-homedir v1.1.0
        github.com/mitchellh/mapstructure v1.4.1 // indirect
-       github.com/natefinch/lumberjack v2.0.0+incompatible // indirect
        github.com/neo4j/neo4j-go-driver/v4 v4.2.4
        github.com/pelletier/go-toml v1.8.1 // indirect
        github.com/rogpeppe/go-internal v1.7.0 // indirect
@@ -32,8 +31,6 @@ require (
        github.com/spf13/pflag v1.0.5
        github.com/spf13/viper v1.7.1
        github.com/tetratelabs/log v0.0.0-20210128123858-0fba4e2dac7d
-       go.uber.org/multierr v1.6.0 // indirect
-       go.uber.org/zap v1.16.0 // indirect
        golang.org/x/crypto v0.0.0-20210317152858-513c2a44f670 // indirect
        golang.org/x/lint v0.0.0-20201208152925-83fdc39ff7b5 // indirect
        golang.org/x/mod v0.4.1 // indirect
@@ -46,3 +43,5 @@ require (
        gopkg.in/ini.v1 v1.62.0 // indirect
        gopkg.in/yaml.v2 v2.4.0
 )
+
+replace github.com/tetratelabs/log => ../log

```

Makefile

@@ -12,7 +12,7 @@ test:
 
 LINTER := bin/golangci-lint
 $(LINTER):
-	wget -O - -q https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b bin v1.23.6
+	wget -O - -q https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b bin v1.38.0
 
 lint: $(LINTER) golangci.yml
 	@echo "--- lint ---"

config.go

@@ -60,11 +60,11 @@ import (
 	"sync/atomic"
 	"time"
 
-	"github.com/natefinch/lumberjack"
 	"go.uber.org/zap"
 	"go.uber.org/zap/zapcore"
 	"go.uber.org/zap/zapgrpc"
 	"google.golang.org/grpc/grpclog"
+	"gopkg.in/natefinch/lumberjack.v2"
 )
 
 // none is used to disable logging output as well as to disable stack tracing.

go.mod

@@ -1,21 +1,12 @@
 module github.com/tetratelabs/log
 
-go 1.15
+go 1.16
 
 require (
-	github.com/BurntSushi/toml v0.3.1 // indirect
-	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/inconshreveable/mousetrap v1.0.0 // indirect
-	github.com/natefinch/lumberjack v0.0.0-20170531160350-a96e63847dc3
-	github.com/pkg/errors v0.8.0 // indirect
-	github.com/pmezard/go-difflib v1.0.0 // indirect
-	github.com/spf13/cobra v0.0.3
-	github.com/spf13/pflag v1.0.3
-	github.com/stretchr/testify v1.2.2 // indirect
-	go.uber.org/atomic v1.3.2 // indirect
-	go.uber.org/multierr v1.1.0 // indirect
-	go.uber.org/zap v1.9.1
-	google.golang.org/grpc v1.15.0
-	gopkg.in/natefinch/lumberjack.v2 v2.0.0 // indirect
-	gopkg.in/yaml.v2 v2.2.1 // indirect
+	github.com/spf13/cobra v1.1.3
+	github.com/spf13/pflag v1.0.5
+	go.uber.org/multierr v1.6.0 // indirect
+	go.uber.org/zap v1.16.0
+	google.golang.org/grpc v1.36.0
+	gopkg.in/natefinch/lumberjack.v2 v2.0.0
 )