fix(tabs): reference parent instead of parentTabs

Author: brandyscarney

src/components/tabs/tab.ts

@@ -233,9 +233,9 @@ export class Tab extends NavController {
 
     parent.add(this);
 
-    if (parentTabs.rootNav) {
-      this._sbEnabled = parentTabs.rootNav.isSwipeBackEnabled();
-    }    
+    if (parent.rootNav) {
+      this._sbEnabled = parent.rootNav.isSwipeBackEnabled();
+    }
 
     this._panelId = 'tabpanel-' + this.id;
     this._btnId = 'tab-' + this.id;