gnu: subversion: Add serf to inputs.

This is a followup to d5e9f380, which upgraded
subversion to 1.8.x.  Previously we used 'neon' to support http access
methods, but subversion 1.8 drops support for 'neon' in favor of 'serf'.

* gnu/packages/version-control.scm (subversion)[inputs]: Add 'serf' and remove
  'neon'.
  [arguments]: Fix typo in phase name.