Remove duplicate dependency

Bundler warns as invalid spec since bundler version 1.5.1.

```
$ bundle install
...blah blah blah
verification at /Users/banyan/foo/.bundle/gems/ruby/2.1.0/bundler/gems/verification-5bb2ffc2c057 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 actionpack (>= 0, development), (< 5.0, >= 3.0.0) use:
    add_runtime_dependency 'actionpack', '>= 0', '< 5.0, >= 3.0.0'
```