fixed gitbook engine version requirement so it works with the current 3.1.1

Author: llusdess

package.json

@@ -1,11 +1,11 @@
 {
   "name": "gitbook-plugin-block-align",
   "description": "A quick way of aligning markdown blocks in a GitBook.",
-  "version": "0.1.2",
+  "version": "0.1.3",
   "author": "Chris Born",
   "license": "Apache-2.0",
   "engines": {
-    "gitbook": ">=3.2.0"
+    "gitbook": ">=3.1.0"
   },
   "main": "index.js",
   "scripts": {