Add source for Gemfile

Without the source, I was getting this error:

```
[~/test/knyle-says][master] 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 rake-10.1.0 in any of the sources
```