set ruby.useBundler config option type to boolean

this allows easier editing from vs code's settings editor

Author: g-arjones

package.json

@@ -146,8 +146,8 @@
           "description": "Method to use for intellisense (go to definition, etc.)."
         },
         "ruby.useBundler": {
-          "type": ["boolean", "null"],
-          "default": null,
+          "type": "boolean",
+          "default": false,
           "description": "Whether ruby tools should be started using Bundler"
         },
         "ruby.pathToBundler": {