Travis CI: re-enable Fontconfig on OS X but force no cache built

Travis CI: re-enable Fontconfig on OS X but force no cache built

Building HarfBuzz from source works to avoid Fontconfig, but it is still
fairly slow. To further speed up the build, try to use only the prebuilt
bottle packages (which inevitably brings in Fontconfig as a dependency)
but hack the Fontconfig formula to avoid building the font cache.

Adding Fontconfig is not the goal of this commit, as we already have it
on Linux and our Fontconfig-related code "should" work equally well on
other platforms. But since we can now afford it, explicitly ask Homebrew
to install Fontconfig even if the dependency that brings it in disappears
from Homebrew in the future, and enjoy the improved code coverage.