Remove testing on mac os

This is failing. It's using python2, which is no longer supported
(though I don't know if that's why it's failing). I don't see the point
in trying to update it to use python3, since I feel we have sufficient
coverage with our linux testing, so for now I'm just removing it.

.travis.yml

@@ -1,7 +1,3 @@
-# NOTE(bja, 2017-11) travis-ci dosen't support python language builds
-#  on mac os. As a work around, we use built-in python on linux, and
-#  declare osx a 'generic' language, and create our own python env.
-
 language: python
 os: linux
 python: 
@@ -11,17 +7,6 @@ python:
   - "3.6"
   - "3.7"
   - "3.8"
-matrix:
-  include:
-  - os: osx
-    language: generic
-    before_install:
-      # NOTE(bja, 2017-11) update is slow, 2.7.12 installed by default, good enough!
-      # - brew update
-      # - brew outdated python2 || brew upgrade python2
-      - pip install virtualenv
-      - virtualenv env -p python2
-      - source env/bin/activate
 install:
   - pip install -r test/requirements.txt
 before_script: