Fix security issue in singularity + misc cleanups (#8657)

- Versions before 2.5.2 suffer from a serious security problem
  (https://nvd.nist.gov/vuln/detail/CVE-2018-12021).  Deleted all
  versions prior to v2.5.2.
- Update homepage (looks like singularity's going Freemium).
- Use release tarball instead of automagically generated archive
  tarball (digest values are more stable).
- No longer need autotools for released versions, just @devel.
- Install needs to be single-threaded, otherwise when installed as
  root the various 'chmod' operations race against the copies.