default `apiProxy.changeOrigin` to true

I ran into this while attempting to proxy connections from my localhost lineman instance to an app running on heroku. Without setting `apiProxy.changeOrigin` to true I just got an unhelpful message from heroku saying 'no such app'.

Spoke with @bjeanes and he suggested this should be the default option as it would be least confusing for people.

Thoughts @jasonkarns @tonyarkles @searls?

https://twitter.com/bjeanes/status/462302173049921536