Up to image build #50496 (where 50000 range means Pharo 5.0) the Pharo images were non-spur. In December the image was migrated to Spur to get more speed and profit from the new virtual machine architecture. So for any newer Pharo 5.0 image you need the new Pharo Spur VM.
PharoLauncher is now able to find out if an image reguires the new Spur VM or the old one. You can set the location of old non-spur VM and new spur VM in the PharoLauncher settings.
This way you can work with old images Pharo 3, Pharo 4, Pharo 5 up to build 50496 as well as with the newer ones.
Read this post for more details or grab PharoLauncher from CI server.