Proc support for config values

Allows something like this:

```ruby
csp = {
  :default_src => 'self',
  :report_uri => lambda {
    if FeatureToggle.available?(:new_csp_endpoint)
      '//example.com/new_csp'
    else
      '//example.com/old_csp'
    end
  }
}
```

(also includes updates to rspec3, removes nonce support, rvm cleanup, etc)

cc @bigloser