Bump version and set as pre-release

Author: zenkyomu

lib/version.js

@@ -1 +1 @@
-module.exports = '2.1.1';
+module.exports = '2.1.2-pre';

package.json

@@ -1,6 +1,6 @@
 {
   "name": "graphlib",
-  "version": "2.1.1",
+  "version": "2.1.2-pre",
   "description": "A directed and undirected multi-graph library",
   "author": "Chris Pettitt <cpettitt@gmail.com>",
   "main": "index.js",
@@ -39,4 +39,4 @@
     "url": "https://github.com/cpettitt/graphlib.git"
   },
   "license": "MIT"
-}
+}