build: drop go Dep, switch to modules, support Go 1.13

bump enumer to v1.1.1
bump golangci-lint to v1.17.1

no `go mod tidy` because 1.13 and 1.12 seem to alter each other's output

fixes #112

daemon/snapper/state_string.go

@@ -4,6 +4,19 @@ package snapper
 
 import "strconv"
 
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[SyncUp-1]
+	_ = x[SyncUpErrWait-2]
+	_ = x[Planning-4]
+	_ = x[Snapshotting-8]
+	_ = x[Waiting-16]
+	_ = x[ErrorWait-32]
+	_ = x[Stopped-64]
+}
+
 const (
 	_State_name_0 = "SyncUpSyncUpErrWait"
 	_State_name_1 = "Planning"