Change-Id: I013dcbfb58ec10756c7b4e4bae6fe79b5e7c0062
Fixes #287

There's no way of serializing/requesting an empty required_contexts that
is needed for bypassing the commit status checks for the ref.

It's a breaking change but will allow to serialize an empty array and
omit empty value as expected.

Deployments API is now official.

https://developer.github.com/changes/2014-11-25-the-deployments-api-is-official/

Fixes #189.