Travis: have only one MacOS build row

[aantron]

I suggest to test only one row, say 4.07, on macOS. This is because macOS testing is the slowest in Travis. I'm not sure if this is true for ocsigen-server, but most projects get full coverage of potential breakage by testing all compiler versions on Linux (to cover the axis of compilers), and one compiler version on macOS (to cover the axis of systems, Linux and macOS). So, it's not necessary to have a full Cartesian product of systems and compilers.

In particular, ocsigen-start's CI is greatly slowing down Lwt's CI by queuing its one macOS build. I guess this is because all repos in the ocsigen org share the same queue in the Travis macOS environment.

I can make a PR if this seems reasonable.