Switch to gometalinter

Only small amount of required checks is enabled,
plan is to enable more linters as issues are fixed in the code.

main.go

@@ -1,8 +1,9 @@
 package main
 
 import (
-	"github.com/smira/aptly/cmd"
 	"os"
+
+	"github.com/smira/aptly/cmd"
 )
 
 func main() {