Run OS X tests in 3 groups in CI.

(cherry picked from commit b44c38930b)

shippable.yml

@@ -15,7 +15,6 @@ matrix:
     - env: T=units/3.5
     - env: T=units/3.6
 
-    - env: T=osx/10.11
     - env: T=rhel/7.4
 
     - env: T=windows/1
@@ -24,6 +23,7 @@ matrix:
 
     - env: T=network
 
+    - env: T=osx/10.11/1
     - env: T=freebsd/10.3-STABLE/1
     - env: T=freebsd/11.0-STABLE/1
     - env: T=linux/centos6/1
@@ -36,6 +36,7 @@ matrix:
     - env: T=linux/ubuntu1604/1
     - env: T=linux/ubuntu1604py3/1
 
+    - env: T=osx/10.11/2
     - env: T=freebsd/10.3-STABLE/2
     - env: T=freebsd/11.0-STABLE/2
     - env: T=linux/centos6/2
@@ -48,6 +49,7 @@ matrix:
     - env: T=linux/ubuntu1604/2
     - env: T=linux/ubuntu1604py3/2
 
+    - env: T=osx/10.11/3
     - env: T=freebsd/10.3-STABLE/3
     - env: T=freebsd/11.0-STABLE/3
     - env: T=linux/centos6/3