ENH: Add the brew cache directory to TravisCI's cache
ENH: Add the brew cache directory to TravisCI's cache

Per

  https://stackoverflow.com/questions/39930171/cache-brew-builds-with-travis-ci

KitwareMedical/ITKUltrasound,KitwareMedical/ITKUltrasound,thewtex/ITKUltrasound,thewtex/ITKUltrasound,KitwareMedical/ITKUltrasound,thewtex/ITKUltrasound