Restore OS X Mavericks builds in TravisCI.

Their stability issues have apparently been resolved, and the backlog is caught up.
We will just add one Mac SKU for now, and take it from there.

.travis.yml

@@ -84,10 +84,10 @@ matrix:
         # OS X Mavericks (10.9)
         # https://en.wikipedia.org/wiki/OS_X_Mavericks
         #
-#        - os: osx
-#          osx_image: beta-xcode6.2
-#          env:
-#              - ZIP_SUFFIX=osx-mavericks
+        - os: osx
+          osx_image: beta-xcode6.2
+          env:
+              - ZIP_SUFFIX=osx-mavericks
 
         # OS X Yosemite (10.10)
         # https://en.wikipedia.org/wiki/OS_X_Yosemite