Capistrano 3.x support

This is an initial pass at Capistrano 3 support. To use:

Add to your `Capfile`:

```ruby
require 'rollbar/capistrano3'
```

And then, to your `deploy.rb`:

```ruby
set :rollbar_token, 'YOUR-TOKEN-GOES-HERE'
set :rollbar_env, Proc.new { fetch :stage }
```