Communicate that v3 is the default version

- Update all mentions of the "default" version to communicate that v3
  is now the default.
- Update the example responses to show the v3 representation.
- Remove the *inline* notes that explained how the v3 representation
  differs from the default representation. (Note that we still provide
  a consolidated list of differences between beta and v3 [1]).

[1]: https://developer.github.com/v3/versions/#differences-from-beta-version
Merge pull request #483 from github/v3-is-the-default-version

v3 is the default version