subversion: Disable serf by default for Linuxbrew
subversion: Disable serf by default for Linuxbrew

Building serf gives the following error:

scons: Reading SConscript files ...
/bin/sh: 1: /usr: Permission denied
OSError: '/usr --cflags gssapi' exited 126:
  File "/tmp/subversion--serf20160224-17504-1skjz6v/serf-1.3.8/SConstruct", line 360:
    env.ParseConfig('$GSSAPI --cflags gssapi')
  File "/home/linuxbrew/.linuxbrew/Cellar/scons/2.4.1/libexec/scons-local/SCons/Environment.py", line 1576:
    return function(self, self.backtick(command))
  File "/home/linuxbrew/.linuxbrew/Cellar/scons/2.4.1/libexec/scons-local/SCons/Environment.py", line 594:
    raise OSError("'%s' exited %d" % (command, status))

Closes Linuxbrew/linuxbrew#182

DoomHammer/linuxbrew,drbenmorgan/linuxbrew,DoomHammer/linuxbrew,Angeldude/linuxbrew,sjackman/linuxbrew,DoomHammer/linuxbrew,Linuxbrew/linuxbrew,sjackman/linuxbrew,Angeldude/linuxbrew,Homebrew/linuxbrew,Linuxbrew/linuxbrew,reelsense/linuxbrew,DoomHammer/linuxbrew,SuperNEMO-DBD/cadfaelbrew,SuperNEMO-DBD/cadfaelbrew,sjackman/linuxbrew,Homebrew/linuxbrew,saketkc/linuxbrew,gnubila-france/linuxbrew,saketkc/linuxbrew,Homebrew/linuxbrew,saketkc/linuxbrew,gnubila-france/linuxbrew,gnubila-france/linuxbrew,saketkc/linuxbrew,Angeldude/linuxbrew,sjackman/linuxbrew,SuperNEMO-DBD/cadfaelbrew,reelsense/linuxbrew,drbenmorgan/linuxbrew,Homebrew/linuxbrew,Linuxbrew/linuxbrew,drbenmorgan/linuxbrew,Linuxbrew/linuxbrew,reelsense/linuxbrew,gnubila-france/linuxbrew,drbenmorgan/linuxbrew,SuperNEMO-DBD/cadfaelbrew,Angeldude/linuxbrew,reelsense/linuxbrew