Define build matrix for this project

[tmatsuo]

@coryan FYI

We're adding kokoro ci builds with #1. Now we're testing Ubuntu 16.04, Ubuntu 18.04, Cent OS 7, and Fedora 30, but we have more to cover. Once we define the build matrix, we'll create issues for it.

Excerpt from #1 (comment)

We certainly want to test gcc-4.8, clang-3.8, ubuntu-18.04 (but also 16.04), we also want to
test the latest and greatest GCC and Clang, we want to test other distros, and macOS, and
Windows.
We want to test producing shared libraries and static libraries too.

[devjgm]

We're moving this project the https://github.com/googleapis/google-cloud-cpp/ monorepo, so this issue will be obsolete at that point since the matrix of builds in that repo will be used.