Use loaded serf version in User-Agent not version used for compilation.

Found by: philip

* subversion/libsvn_ra_serf/ra_serf.h
  (USER_AGENT): Remove.

* subversion/libsvn_ra_serf/serf.c
  (get_user_agent_string): New. Use serf_lib_version() to get serf version.
  (svn_ra_serf__open): Use get_user_agent_string() function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1514315 13f79535-47bb-0310-9956-ffa450edef68