Fix for broken librato output (#2225)

* Fix for broken librato output

These errors are delightful, but I'd rather avoid them:

```
Error parsing /etc/telegraf/telegraf.conf, line 2: field corresponding to `api_user' is not defined in `*librato.Librato'
```

* Fixed bad format from last commit

plugins/outputs/librato/librato.go

@@ -17,8 +17,8 @@ import (
 
 // Librato structure for configuration and client
 type Librato struct {
-	APIUser   string
-	APIToken  string
+	APIUser   string `toml:"api_user"`
+	APIToken  string `toml:"api_token"`
 	Debug     bool
 	SourceTag string // Deprecated, keeping for backward-compatibility
 	Timeout   internal.Duration