Travis: use xdist for macos builds

Author: blueyed

.travis.yml

@@ -52,9 +52,9 @@ jobs:
       sudo: required
       install:
         - python -m pip install --pre tox
-      env: TOXENV=py27-lsof
+      env: TOXENV=py27-xdist
     - <<: *test-macos
-      env: TOXENV=py37-lsof
+      env: TOXENV=py37-xdist
       before_install:
         - brew update
         - brew upgrade python