Issue 794: Tavis CI timeout on brew install
Issue 794: Tavis CI timeout on brew install

Travis uses an old MacOS version by default (10.13) and this is not supported
by brew. We should explicitly set a more recent `osx_image`.

equalsraf/neovim-qt,equalsraf/neovim-qt,equalsraf/neovim-qt,equalsraf/neovim-qt