Update PreviousNext when navigating to a new page

Earlier, it was just computing it once and then leaving it there
permanently. So you'd get "recommends" for the first post you
opened rather than the current one, unless you went to a non-post
page in the middle or reloaded the page.

src/components/PreviousNext.vue

@@ -89,6 +89,7 @@ export default {
   },
   methods: {
     getPreviousNext() {
+      this.PreviousNexts = []
       const allBlogs = this.$static.allGhostPost.edges;
       for (let i = 0; i < allBlogs.length; i++) {
         if (allBlogs[i].node.id === this.currentPostId) {
@@ -101,6 +102,13 @@ export default {
         }
       }
     }
+  },
+  watch: {
+    id: function(val) {
+    console.log('Updating PreviousNext')
+      this.currentPostId = val
+      this.getPreviousNext()
+    }
   }
 };
 </script>