disable travis mac builders and add a mac jenkins build (#3933)

we'll re-enable mac builders when travis recovers and remove the jenkins
build
diff --git a/Jenkinsfile b/Jenkinsfile
index 8742e85..92b0c7e 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -17,7 +17,7 @@
     ],
     [
         label: 'sierra',
-        toxenvs: ['py27'],
+        toxenvs: ['py27', 'py36'],
     ],
     [
         label: 'yosemite',