[travis] build/test release versions first

.travis.yml

 
 matrix:
   include:
-    - os: osx
-      r: release
-    - os: osx
-      r: oldrel
-    - os: linux
-      r: devel
     - os: linux
       r: release
       env:
       - BUILD_PKGDOWN=true
+    - os: osx
+      r: release
+    - os: linux
+      r: devel
     - os: linux
       r: oldrel
+    - os: osx
+      r: oldrel
 
 # meta -------------------------------------------------------------------------
 language: r