Bump required serf version to 0.7.1, to prevent various memory and stability Badness.
Bump required serf version to 0.7.1, to prevent various memory and
stability Badness.

* configure.ac:
  Minimum serf: 0.7.1

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion