Switch to VXL master branch.

Our VXL revision temporarily pointed to a fork of VXL while we were waiting for PRs to be accepted. They have now been accepted, so we can start tracking the main fork of the project.

versions.cmake

@@ -60,12 +60,9 @@ superbuild_set_revision(cmbworkflows
   GIT_TAG        origin/master)
 
 superbuild_set_revision(vxl
-# This fork of https://github.com/vxl/vxl has two updates whose pull requests
-# are pending on the main repository (https://github.com/vxl/vxl/pull/485,
-# https://github.com/vxl/vxl/pull/486). Once they are accepted, we can change
-# this depedency to vxl's master branch.
-GIT_REPOSITORY "https://github.com/tjcorona/vxl"
-GIT_TAG "for/smtk")
+# VXL master branch @ May 29, 2018
+GIT_REPOSITORY "https://github.com/vxl/vxl"
+GIT_TAG 4df45773ea8a74d3560b55a05d5fbb2f76741aac)
 
 # Use opencv from Thu Oct 6 13:40:33 2016 +0000
 superbuild_set_revision(opencv