Only run mac on one configuration

It's by far the slowest to run – a sniff test should be sufficient

Author: kevmoo

.travis.yml

@@ -12,9 +12,14 @@ env:
 script: ./tool/travis.sh
 
 os:
-  - osx
   - linux
 
+matrix:
+  include:
+  - env: DARTDOC_BOT=main
+    os: osx
+    dart: "dev/raw/latest"
+
 install:
 - ./tool/install_travis.sh