Revert "Update POM version numbers to 5.1.0-m1"

This reverts commit edc3c5296d50e99cf08f608dd7208c45302ec76c so that POM version numbers are ```5.1.0-SNAPSHOT```.  This should be merged before any other develop PRs go in - I mistakenly merged gh-1329 forgetting that this hadn't been done yet.