Bump version and set as pre-release

Author: rustedgrail

lib/version.ts

@@ -1 +1 @@
-export const version = "3.1.1";
+export const version = "3.1.2-pre";

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@dagrejs/dagre",
-  "version": "3.1.1",
+  "version": "3.1.2-pre",
   "description": "Graph layout for JavaScript",
   "author": "Chris Pettitt <cpettitt@gmail.com>",
   "contributors": [
@@ -67,4 +67,4 @@
     "type": "git",
     "url": "https://github.com/dagrejs/dagre.git"
   }
-}
+}