BUG: require cmake 2.8.11 for testing

The requirement of ExternalData, was first released with cmake
2.8.11. This is the required version to enable testing. Check the
cmake version to intelligently enable testing, if the build is capable.

Change-Id: Iecc8c7b4c0ae03e4bd40cf9d66c683a6bfbb2815
Merge topic 'FixCMakeRequiredVersionForTesting' into next

cf9f91c BUG: require cmake 2.8.11 for testing