fix: adapt to new unkown version marker

Follows 7a0d18ceb6.

cli/app/rollback.go

@@ -85,8 +85,7 @@ recipes.
 		}
 
 		var availableDowngrades []string
-		if deployedVersion == "" {
-			deployedVersion = "unknown"
+		if deployedVersion == "unknown" {
 			availableDowngrades = versions
 			logrus.Warnf("failed to determine version of deployed %s", app.Name)
 		}

cli/app/upgrade.go

@@ -92,8 +92,7 @@ recipes.
 		}
 
 		var availableUpgrades []string
-		if deployedVersion == "" {
-			deployedVersion = "unknown"
+		if deployedVersion == "uknown" {
 			availableUpgrades = versions
 			logrus.Warnf("failed to determine version of deployed %s", app.Name)
 		}

cli/app/version.go

@@ -56,7 +56,7 @@ Cloud recipe version.
 			logrus.Fatal(err)
 		}
 
-		if deployedVersion == "" {
+		if deployedVersion == "unknown" {
 			logrus.Fatalf("failed to determine version of deployed %s", app.Name)
 		}