travis.yml: tweak settings.
travis.yml: tweak settings.

General cleanup and use only one OS X worker (as these workers are shared between the Homebrew organisation).

Closes #370.

Signed-off-by: Alex Dunn <46952c1342d5ce1a4f5689855eda3c9064bca8f6@gmail.com>

Homebrew/homebrew-emacs