Update required for Gibbon 2.0

Gibbon 2.0.0 was released July 29, 2015.

When you run `bundle install` or `bundle update` the newest Gibbon 2.0 gem will be installed and the application will error with

```
NameError: uninitialized constant Gibbon::API
```

Either change the Gemfile and specify 

```
gem 'gibbon',  '~>1.2.0'
```

or update the code that uses Gibbon.

See https://github.com/amro/gibbon/issues/131
