ci/macOS: skip python2 on travis macOS

macOS travis builds recently started failing (travis caches were cleared
recently, maybe related). python2 is reasonably covered by linux CI. Not
going to waste time on it for macOS CI.

    ==> Installing python@2
    ==> Downloading https://homebrew.bintray.com/bottles/python@2-2.7.14_3.el_capita
    ==> Pouring python@2-2.7.14_3.el_capitan.bottle.tar.gz
    Error: The `brew link` step did not complete successfully
    The formula built, but is not symlinked into /usr/local
    Could not symlink bin/2to3-2
    Target /usr/local/bin/2to3-2
    is a symlink belonging to python. You can unlink it:
      brew unlink python
    To force the link and overwrite all conflicting files:
      brew link --overwrite python@2
    To list all files that would be deleted:
      brew link --overwrite --dry-run python@2
    Possible conflicting files are:
    /usr/local/bin/2to3-2 -> /usr/local/Cellar/python/2.7.12_1/bin/2to3-2
    /usr/local/bin/2to3-2.7 -> /usr/local/Cellar/python/2.7.12_1/bin/2to3-2.7
    /usr/local/bin/idle -> /usr/local/Cellar/python/2.7.12_1/bin/idle
    ...

0 files changed, 0 insertions, 0 deletions