Fix `Unsupported version control system "svn"` errror on some system, the reason is the that other version of SVN dlls in system PATH.

detail:
{{{
Warning: Can't synchronize with repository "(default)" (Unsupported version control system "svn": DLL load failed: ²Ù×÷ÏµÍ³ÎÞ·¨ÔËÐÐ %1¡£). Look in the Trac log for more information. 
}}}

ref: http://trac.edgewall.org/ticket/7682