fix(MdTooltip): mdActive never updated to true using .sync modifier

`mdActive` was never updated to true via `.sync` before

src/components/MdTooltip/MdTooltip.vue

@@ -55,14 +55,16 @@
     watch: {
       mdActive () {
         this.shouldRender = this.mdActive
+      },
+      shouldRender (shouldRender) {
+        this.$emit('update:mdActive', shouldRender)
       }
     },
     methods: {
       show () {
         this.shouldRender = true
       },
       hide () {
-        this.$emit('update:mdActive', false)
         this.shouldRender = false
       }
     },