Both of those endpoints use PUT method, which means they must send
the parameters as a JSON encoded body rather than query parameters.

Source: https://developer.github.com/v3/#parameters

Fixes #402.
Closes #403.

Change-Id: I444d1ce9ba881200282ff617454addc2e6eb4793

Note that this is an API-breaking change but should have minimal
impact on users of this package due to the nice inference
properties of the Go programming language.

Bumped `libraryVersion` to `2` due to API-breaking change as
discussed in #376.

Fixes #180.

Change-Id: Ib386135e6b8f306d1f54278968c576f3ceccc4e7