Fix version numbers and release dates. (googleapis#1578)

The version numbers in google/cloud/CMakeLists.txt was off by one. Also
fixed the release date in the release notes.

Make ccache bigger for coverage builds. (googleapis#1567)

We need a bigger cache for the code coverage build. While most builds
take 500MiB, the code coverage build takes nearly 1.5GiB.

When I ran this the build time went from ~37m to ~30m, that is a big gain,
and the coverage build is the longest build so it speeds up the iterations.

Create v0.4.x release branch

Update release notes. (googleapis#1379)

Label branch as release branch for build purposes.

Implement future<void> and promise<void>. (googleapis#1367)

This implements futures and promises for `void`. It does not yet
implement the `.then()` member function, to keep the PR size small. Part
of the work for googleapis#1345.

Update v0.2.x release notes [skip ci]. (googleapis#964)

Update v0.2.x release notes [skip ci]. (googleapis#964)

Update README files to declare this is an Alpha release. (googleapis#385

)

Integration test for InstanceAdmin::ListInstances. (googleapis#371)

This fixes googleapis#343.