Gemfile should specify at least one gem source

Otherwise the instructions to "bundle install" are kinda worthless.

```
$ bundle install
Your Gemfile has no gem server sources. If you need gems that are not already on your machine, add a line like this to your Gemfile:
source 'https://rubygems.org'
Could not find multi_xml-0.5.5 in any of the sources
[1]    50033 exit 7     bundle install
```