Sort imports before serializing lock

No more superfluous lock changes...let's hope
diff --git a/cfg/lock.go b/cfg/lock.go
index a40bada..dcd3185 100644
--- a/cfg/lock.go
+++ b/cfg/lock.go
@@ -67,6 +67,8 @@
 
 // Marshal converts a Config instance to YAML
 func (lf *Lockfile) Marshal() ([]byte, error) {
+	sort.Sort(lf.Imports)
+	sort.Sort(lf.DevImports)
 	yml, err := yaml.Marshal(&lf)
 	if err != nil {
 		return []byte{}, err