Delete dup dependency declaration

In order to fix warn:
```
airbrake at /home/vagrant/src/shopify/vendor/bundle/bundler/gems/airbrake-cb0b6ff0bb5d did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on activesupport (~> 2.3.8, development), (>= 0) use:
    add_runtime_dependency 'activesupport', '~> 2.3.8', '>= 0'
```