As discussed recently in several other PRs.
This bumps the build SDK to 10.9. I would bump to 10.10, but our current clang has problems there, and I suspect that will eventually fix itself when we bump to a newer build distro for gitian/travis. The 10.10 bump can come at that point.
This also bumps the minimum requirement to 10.7, leaving 10.6 no longer supported. This is in line with what other applications are doing these days as well. 10.7 is the first version that no longer supports x86 (x86_64 only), as well as supporting libc++, leaving that as an option for us in the near future.
Tested locally and via Gitian with no problems. All font/unicode issues are resolved as expected.
This also bumps the version used for Travis builds, so this PR itself will build against 10.9.
